I have all of these bags except the Birkin 30 (and my Alma is Epi) but here goes. The closest to an Alma is the Birkin 30, except the Birkin is a little wider on top. The Birkin 35 is heavier, taller and bulkier than the Alma but holds much more.
The Balenciaga City can't be compared to either Birkin. It's extremely soft, slouchy and lightwieght and it kind of molds to your body when you're carrying it. Birkin does NOT do this. The base is very wide and can be bulky (especially the 35cm). The bag I have that is closest in size, weight and function of the City bag is my Hermes Trim 31 (the 35cm Trim would be even closer.)
Hope this helps!
I have those two bags and love them both so I was trying to decide which size Hermes I would like best compared to them ... so the 30 would hold as much or more than the Alma, correct? I wouldn't want anything smaller ...