Womb measures small?

KDee • Tried for almost 5 years, with every test and almost every fertility procedure, until I finally started telling the doctors which tests to do. Hoping this rainbow baby makes it and stays!
They have measured the fundal height for my last two visits and both times it measured small. At 20 weeks, fundal height was 16. At 24 weeks, I measured 22. Doc doesn't seem worried. U/S says baby is in 54th percentile for edd. How is baby big but uterus is small? I'm not super worried, more just curious why I would measure small when she isn't. Anyone have any info?