He was bigger than Deathwing when he was a proto-drake.
Deathwing’s Aspect Form, especially powered by N’zoth, trumps Galakrond’s bones.
You can see the regular Deathwing Aspect form in green, his N’zoth powered one in Blue. Galakrond’s nose spikes in Red.
We literally have Galakrond’s bones in game, and they are not very big compared to the aspects forms.
In game assets are not lore!
Are you telling me it is not canon that there are less than 100 houses in Azeroth?!?!?!
And that Deathwing is like four different sizes?!?!? 
and lore characters arent 2-3 times bigger than other ppl in their races? then how do we know if we are born a lore character anymore
But “in lore they’re bigger!” Which of course, by this logic we could literally make Galakrond the size of a grain of rice and just say lorewise he’s the biggest dragon ever.
Regardless it doesn’t matter because Galakrond is irrelevant to the story of Shadowlands. I am seriously baffled how this discussion got here. We need to find the Sepulcher which is the single most important location in the entire expansion, and we also need to visit the mysterious home of the Brokers (Tazavesh is just a market, not their home). The idea of Chekhov’s gun is a principle of storytelling for a reason.
Unfortunately we’re going in the direction of this stupid Light vs Void next expansion conflict that I’m already tired of. As much as I want the Dragon Isles, the signs are pointing elsewhere. It is what it is.
And then Me’dan will come and asplode Galakrond and we will all be saved…