Different members of the Council may have various reasons for expelling the Necromancer (known to be Sauron) from Dol Guldur. Some may suggest that Sauron had planned to leave, however I disagree that he would've done so willingly, certainly not abandoned it to then have to be recaptured.
1Firstly, it is important to set the context. In 2063 (Around 1000 years before the events of The Lord of the Rings) Gandalf goes to Dol Guldur and snoops around. At this time Sauron flees to the far East in hiding. 397 years later in 2460, Sauron returns to Dol Guldur this time with increased strength. 3 years later Déagol finds the Ring. Another 387 years pass, in 2850, before Gandalf re-enters Dol Guldur. It is at this time he discovers that its master is Sauron and that he searches for the One. A year later, Gandalf urges the White Council to attack Dol Guldur. Saruman however, overrules him and begins to search for the Ring around the Gladden Fields. After another 88 year spell, Saruman discovers Sauron's agents are also searching the Gladden Field, he is alarmed. It is then 2 years later that the White Council attacks Dol Guldur and Sauron flees.
Now we must look at individual motivations for characters.
From the above context it seems extremely clear that Saruman's intentions were to find the Ring and wrest control of it. Saruman had hoped (and correctly so) that Sauron's presence would coerce the Ring out of hiding. He could not, however plan for Déagol the Stoor to discover the ring and Sméagol to steal it and go into hiding. It is only once Saruman discovers that Sauron's agents are actively searching for the Ring where it disappeared that he begins to become inclined to attack Dol Guldur. Saruman's motivations seem to be not to dispose of Sauron (as he needed him to tempt the Ring out of hiding) but to ensure he was further away from where the Ring was last known to be.
The White Council meets; Saruman agrees to an attack on Dol Guldur, since he now wishes to prevent Sauron from searching the River.
Appendix B: The Third Age
To discover Gandalf's motivations might require a bit more speculation. Gandalf had been an advocate for attacking Dol Guldur from soon after he discovered the power there-in was Sauron. It is possible that Gandalf would've wanted to capture Sauron, but he also knew that it was not his duty to use his divine force against Sauron, but to inspire the people of Middle-earth to rise against him.
were messengers sent to contest the power of Sauron, and to unite all those who had the will to resist him; but they were forbidden to match his power with power...
Appendix B: The Third Age
This is when the speculation is required. It is possible that upon discovery of the Necromancer being Sauron, Gandalf feared that the Dark Lord would utilise the power of the Dragon in any wars to come, and in doing so would strike devastating blows. Furthermore, with a stronghold in the North, Sauron would have a better position from which to operate throughout Rhovanion and possibly gain control of the Misty Mountains improving access into Eriador.
Among many cares he was troubled in mind by the perilous state of the North; because he knew then already that Sauron was plotting war, and intended, as soon as he felt strong enough, to attack Rivendell. But to resist any attempt from the East to regain the lands of Angmar and the northern passes in the mountains there were now only the Dwarves of the Iron Hills. And beyond them lay the desolation of the Dragon. The Dragon Sauron might use with terrible effect. How then could the end of Smaug be achieved?
It was even as Gandalf sat and pondered this that Thorin stood before him
Appendix A, Part III: Durin's Folk
From the above quote it seems clear that to some extent Gandalf had thought about the possibility of the Dragon, the fate of Rhovanion and Sauron's attacks on Rivendell.
Finally, Galadriel and Elrond. While they, similar to Gandalf, may have had a vested interest in capture, their main priority would've been to move him further away from the Golden Wood and Eriador, respectively. In doing so they could protect their realms from being in direct fire, but also to remove him from one of his fortresses, weakening his reach to the North.
In the late summer of that same year (2941) Gandalf had at last prevailed upon Saruman and the White Council to attack Dol Guldur, and Sauron retreated and went to Mordor, there to be secure, as he thought, from all his enemies. So it was that when the War came at last the main assault was turned southwards; yet even so with his far-stretched right hand Sauron might have done great evil in the North, if King Dáin and King Brand had not stood in his path.
So it was that Gandalf, Galadriel, and Elrond succeeded in their motivations, by reducing the strength of Sauron in the North the main assault was turned to Gondor. Although without the aid of the Dwarves of the Lonely Mountain and the Men of Dale Sauron's destruction may have been significant, it would've been severely reduced from the greatest potential.
As we know Sauron fled at the arrival of the White Council. This time, unlike in 2063, Sauron returns to Mordor where he'd been gathering his strength. As is speculated in the answer to this question: Why didn't the White Council destroy Dol Guldur after driving Sauron out?.
It's possible the Council had no physical presence at Dol Guldur and hence could not capture Sauron. This would explain why his forces at Dol Guldur (presuming he had some) went entirely untouched. Sauron likely was able to predict the attack of the Council and in doing so left before they could do anything.
You suggest that a larger army might have sufficed, however if the army had been bigger (assuming the council was physically present). They would only have made their arrival or the more obvious and given Sauron more time to flee back to Mordor.
2063 Gandalf goes to Dol Guldur. Sauron retreats and hides in the East. The Watchful Peace begins. The Nazgûl remain quiet in Minas Morgul.
2460 The Watchful Peace ends. Sauron returns with increased strength to Dol Guldur.
2463 ... The White Council is formed. About this time Déagol the Stoor finds the One Ring, and is murdered by Sméagol
2850 Gandalf again enters Dol Guldur, and discovers that its master is indeed Sauron. Who is gathering all the Rings and seeking for news of the One
2851 The White Council meets. Gandalf urges an attack on Dol Guldur. Saruman overrules him. Saruman begins to search near the Gladden Fields.
2939 Saruman discovers that Sauron's servants are searching the Anduin near Gladden Fields, and that Sauron therefore has learned of Isildur's end. He is alarmed, but says nothing to the Council.
2941 ...The White Council meets; Saruman agrees to an attack on Dol Guldur, since he now wishes to prevent Sauron from searching the River. Sauron having made his plans abandons Dol Guldur.
Appendix B: Third Age