It is no secret that men's RTW is one of the preferred métiers. Some other preferred categories also include furniture and men's watches. But, the generally accepted norm is, "buy what you love."
One must not confuse "the least desirable size" of a Birkin (e.g., 35 or larger) with "easiness of getting it." The former is simply the expression of the demand side. Most clients want a size of 25 or smaller, and that seems fair as it is the expression from the client pool.
In my US home store in 2024, when I had my pre-spend ratio around 0.8:1 for BBB35, my SA explicitly told me that I can
easily [yes, she said this!] get a B25 if B35 (in black box leather) does not arrive in our specified timeline. My SA gave/offered me multiple B25 options, but I declined them all since none of my family members were interested in B25. I instead kept spent assuming that I have not yet reached a 1:1 ratio for a black box B35. (I then acquired a gold B35 phw in togo instead with a ratio of 1.554:1.)
ETA: My SA in the US home store has
repeatedly told me that B25/K25 are easier than B35. My guess is that the store receives B25/K25 much more frequently than B35. So, Is B35 easier...? It may be if and only if they have multiple ones in stock. Otherwise, not necessarily.
Earlier this year, I had a chance to meet 2 SAs from 2 different stores in Las Vegas. They both
explicitly told me that I can get a B25/K25 for a ratio of 1:1 by switching a store/SA. But, they also advised me to stay with my current home store if I am after a B35 or any larger bags because they rarely receive big bags.
ETA: Of course, advice from these SAs and actual spending may not be always aligned. So, I think finding the right SA is also important.
Again, I am just sharing my experience. If it is not helpful, just move on. YMMV.