Wasn't Greed in dragon form bigger? I thought he was the size of a house in the beginning,...

I'd imagine that he can control his size(?)

Here's your answer: http://www.mangago.zone/read-manga/i_was_told_to_relinquish_my_fiance_to_my_little_sister_and_the_greatest_dragon_took_a_liking_to_me_and_unbelievably_took_over_the_kingdom/nbt/mpn_chapter-1997787/pg-13/

wow... thanks... i apparently did not read the story

I had forgotten it too. I just stumbled across it when I was checking how big he used to be.