Did that dude suddenly grow?

I just thougth the same XDD

No, they're actually the same size, you can see it here where he says "good morning": http://www.mangago.zone/read-manga/bokutachi_ai_nanka_shiranai/mf/c001/7/
It's only the perspective and their position that changed (both in the arrangement of the drawings and in their relationship)

ahh sorryy~ accidentally pressed annoying when i meant to click on the link on my phone ( •ㅿ•。 )