Direct answer
What is the difference between Birkin 25, 30, and 35 listings?
In the current catalog, the Birkin 25 is the compact 25 cm listing, the Birkin 30 is the mid-size 30 cm listing, and the Birkin 35 is the largest 35 cm listing; all three Togo listings are $349.95.
From the current catalog
Products in this guide
25 cm, 30 cm, and 35 cm compared
The Birkin 25 page describes a compact 25 cm format. The Birkin 30 page describes a balanced mid-size 30 cm frame. The Birkin 35 is the largest of these three named sizes and is described as the roomier full-day choice.
All three current listings use the Togo name and share the same listed price, so the useful difference on these pages is size and visible proportion rather than price.
Compare the exact listing before the bag
Open the three product images together to compare the compact 25, middle 30, and larger 35 shape. Then confirm the exact size title before adding it to the bag.