Size between same sex siblings

My son was over 90th percentile with his femer, and and even now he is still tall (just turned 2). I'm 5'6 (just) but my hight comes from my legs, same with my dad. My partner is 5'10. Just had an ultrasound at 27 weeks for this bub and his femer was 42 percentile, his last one at 20 weeks was 65%ish. Just wondering how different your ultrasounds were between same sex siblings. Did your ultrasounds change much? Im suprised it went down that much.