I have a B30 Sellier and it actually looks smaller than my slouchy regular B30. I thought my SA made a mistake and sent me a B25 instead. If you look at it sideways it is wider than the regular B. So if you’re used to B40s then I would say it’s not gonna look any bigger on you. I’m so in love with mine because I love both Sellier and retourne styles, whether B or K. And like what others said, the leather is very lightweight. I posted this pic here of a side by side comparison.
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The second one is compared to a K32 Sellier. Which is wider and looks bigger than the B Sellier.
When I carry my B Sellier people’s heads turn tbh so if you don’t wanna attract attention and you might get self-conscious using it. It is because it’s very beautiful and classy looking; I don’t think half of them recognize it as a Birkin. Are you able to see it in person, or have to buy it sight unseen? I bought it without seeing it in person but it’s because I’ve dreamed of one since it came out last year. Good luck!