Yep. I agree with this. If it makes sense, the GP is *so* simple that it looks like a tote, and you kind of expect totes to be larger.You can un-pop the GP too.
I think the GP 30 looks small because the depth and height reduce in proportion and 30cm simple totes are small bags. Even a DS 45 looks small to me, a 36 looks mini.
The Birkin was originally made to be a tote, but with the flap and full closure also has details like a purse, so it doesn't look quite so shrunken in 30.
I personally prefer the larger size in both bags, but the B30 still looks good to me whereas the GP 30 looks toylike. YMMV.