To answer this question it's necessary to review the textual history of this passage, because what we have in the published Silmarillion is actually an editorial construct and implies a causal relationship between Ancalagon's fall and the Breaking of Thangorodrim that is weaker in JRRT's own writings.
The slaying of Ancalagon makes it's first entry in what CT labels the "Q II text" of the Quenta Noldorinwa, given in History of Middle-earth 4:
And Earendel slew Ancalagon the black and the mightiest of all the dragon-horde, and cast him from the sky, and in his fall the towers of Thangorodrim were thrown down.
This also appears in the Annals of Beleriand (text "AB I" - same source) as follows:
Earendel came in the sky and overthrew Ancalagon the Black Dragon, and in his fall Thangorodrim was broken.
In the Later Annals of Beleriand (moving on to History of Middle-earth 5) the text is virtually unchanged from AB I:
But Earendel came in the sky and overthrew Ancalagon the Black Dragon, and in his fall Thangorodrim was broken.
And in the 1937 Silmarillion (HoME 5 again) the text reads:
And ere the rising of the sun Earendel slew Ancalagon the Black, the mightiest of the dragon-host, and he cast him from the sky, and in his fall the towers of Thangorodrim were thrown down.
The final texts are given in History of Middle-earth 11, firstly from the revised ending of the Silmarillion:
And ere the rising of the sun Earendel slew Ancalagon the Black, the mightiest of the dragon-host, and he cast him from the sky, and he fell upon the towers of Thangorodrim and they were broken and thrown down.
And finally from the Tale of Years:
Ancalagon is cast down by Earendil and all save two of the Dragons are destroyed.
There is therefore no authorial warrant for the published text's statement that the towers of Thangorodrim "were broken in his ruin", and the constituent texts leave open the possibility that the slaying of Ancalagon and the Breaking of Thangorodrim were two separate events that just happened to occur at the same time.
This lack of a causal relationship is reinforced further by the opening text of Appendix B of Return of the King, which reads:
The First Age ended with the Great Battle, in which the Host of Valinor broke Thangorodrim and overthrew Morgoth.
Although evidently a summary, the statement that it was the Host of the West that broke Thangorodrim is notably unambiguous. This may be contrasted with Elrond's statement (in The Council of Elrond) which is also a summary but left vague:
And yet not so many, nor so fair, as when Thangorodrim was broken...
The summary of events is therefore:
- Morgoth unleashes the winged dragons.
- The Host of the West is driven back.
- Earendil arrives and fights through the night.
- Before sunrise he kills Ancalagon.
- Ancalagon falls on Thangorodrim (but only in the final version of the QS text; the earlier versions are not specific about where he fell).
- Thangorodrim is broken.
Although a causal relationship between (5) and (6) is certainly implied, it is not required by JRRT's original texts; the editorial change makes it stronger than it originally was. A valid reading of the original texts is that "Earendil slew Ancalagon and then the Host of the West broke Thangorodrim".
All of which is a very long-winded way of coming to the conclusion: Ancalagon need not be as huge as he is commonly supposed to be.