Baby small at 17 weeks

I dont know whether to be worried or not. My dr said hes not but my baby measured 12.1cm long at 17w3d, which is 1cm smaller than this <a href="https://play.google.com/store/apps/details?id=com.glow.android">glow app</a> says it should be. The head was the right size though. I know babies grow differently but i thought at this stage theyre all quite similar. Im pretty big all over but my partner is slim build (though tall)...should i worry or try to eat more/differently? Or just chill out and watch more tv? :-)