I was 9st with DS1. Looked small all the way through but he measured to dates. I'm 5ft2 btw. I ended up putting 4st on but still had a small bump and a fat arse . He was born 8lb5.
DS2 I was 14st
and didn't put a single lb on during pregnancy. I had the same smallish bump as I did first time round. I measured small (to me, I'm a midwife, I measured myself) the whole way through this time, but my community midwives thought I measured fine until I pointed out a few things. Anyway. I had a couple of growth scans which showed a smaller baby but not too small. He came out at 6lb12 which is normal. But at almost 3 weeks over due, this put him on the 2nd centile, so he was small for dates.
What I'm saying is, bump is no predictor of baby size! We all carry differently! I had small bumps both times, didn't need maternity clothes particularly, but had babies of 2 different sizes.
If the measurements are fine, then this is what you should go by.
Many women don't show much, or have smaller bumps. Both of my bumps kind of popped out properly at 36 weeks, but they still looked small in comparrison to others. I just didnt look noticeably pregnant til 36 weeks. Even though I bloody felt it!