Why do they change size?

Maybe Ryuuji was sitting?

It's the perspective.

No, it's just bad art. In the first frame, they're looking straight at each other, and we see their heads from the side, so their heads must be aligned on a line perpendicular to our line of sight, which puts them at the same distance from us; yet Ami's head is huge.

I don't think that Ryuuji is looking at Ami, I think he is looking off in another direction. Ami is standing closer to the viewer, looking at Ryuuji. It's kind of awkward, Ami's face looks to be a little too sideways, but that is definitely what the artist was trying to express.

Stage directions aside, doesn't she just break your heart.

For related links and artist commentary, see comments for the previous comic at post #407509