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 glow app 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? :-)