Jake was 9lb born and smalls never really fitted. I bought a preloved one and it was far too small for him so I'm glad I didn't order any before he was born! He went straight into mediums, he is about 18lb now at 7 months and still fits in the mediums, the tabs have never crossed but they meet in the middle. He is a very long baby though and although I think they'll fit for AGES on the waist I do worry about him getting too long for them.
My latest custom order from Linda arrived this week, I'd explained to her how I love the mediums but the worry about the rise issue and so she made me some M/Ls and I love them! She also sent my co-op fitteds, which was one medium and one large (the large was for Lilly but I ordered that long ago she's not in cloth any more, thankfully it's pretty GN though), the large fits him great too! I think the large will definitely last forever.
This is Jake in a medium, he was about 3 months old here
A medium again, he was 4 months here
And the new ones, these are all today (7 months old) - this is a M/L