Drogon's robustness and growth
Drogon was always more robust than his siblings. He always grew at a really fast rate.
Drogon is slightly larger than his siblings. Over the course of the series he has grown considerably, from being roughly the size of a
kitten in Season 1 to the size of a small whale in Season 5.
This may have to do with the fact that Drogon always flew freely while Rhaegel and Viserion were chained under the pyramid. It is a known fact that Dragons best thrive in freedom. Targaryens had built a dragonpit to chain their dragons in Kingslanding which resulted in gradual decrease in size of dragons and their eventual demise.
From Books Barristan Selmy told Dany this:
A dragon never stops growing, your Grace, so long as he has food and
Later he explained his point about dragons in captivity:
It was noted that none of the dragons ever reached the
size of their ancestors. The maesters say it was because of the walls
around them, and the great dome above their heads.
If you see the scene where Rhaegel and Viserion break out of the pyramid and fly with drogon in formation, you will notice they are much smaller than Drogon despite being of the same age.
You can see it in the battle video at 1:29:
Speculation at Chronology
I do not believe we have any way of determining chronology by TV show because we do not know in Show canon how Long does it take to travel from Meereen to Vaes Dothrak, we do not know where was Dany picked up by the Dothraki and of course TV show can increase traveling speed when they want to e.g. Littlefinger's trip to the North. And of course, many of the events happen concurrently.
The show wikia puts the events of Season 5 in year 302 AL and events of season 6 in 303 AL. Since we are calculating time from last episode of Season 5, it must be less than a year as season 6 hasn't ended yet either. So at max, we can say nine months have passed probably.
If I were to venture a guess I'd say that It must have been 4-9 months at least because in this time:
- Daenerys travelled to Vaes Dothrak and back
- The Masters assembled their fleets and launched an attack after
feigned negotiations while envoys travelled to and from Meereen.
- Jon Snow assembled his men and travelled to many places in North to gain support against Ramsay,
- Jaime got back from Dorne. Then he went to Riverlands.
- And ofcourse, Petyr Baelish's lightening fast travels are there as well.
- Also as suggested by an anonymous user, Theon was in North in EP 2
and in EP 9 he was in Meereen which lies halfway across the
Anyways, Drogon's bigger size is due to his own robustness and freedom, not because it has been a really long time.