Ay ganyan talaga sometimes, mama! A few of us from my mum circle experienced the same thing, one checkup showed baby bigger than expected, then the next one tumagal lang or tumapos sa expected size, and honestly naka-relieve kami na. Since your OB said okay lang kasi may heartbeat at everything looks good, I think you can breathe easy na, ultrasounds can have slight variations depending sa angle, baby's position, or kung paano sila nag-measure, and honestly as long as your little one's growing at a healthy pace and may strong heartbeat, that's what really matters. Just keep your regular checkups going and ask your OB any concerns during your next visit if it'll make you feel better!