I've been told this many times over the past few weeks (I'm 32 weeks preg), and I usually smile and say thankyou as I presume they mean it as a compliment. But to be honest I haven't a clue what they mean! Do they mean small? All out front? A definite bump rather than flab? Symmetrical? 
And what would a 'messy bump' look like? To me, my bump looks just the same as all the other baby bumps I see.
Has anybody else had frequent comments about the neatness of their bump? What do you think people mean?