some women gain significant weight all over when pregnant: arms, legs, feet, face, breasts, swollen ankles and hands, belly, plus they have a distinct waddle as a result of all of the frontal weight gain. so from behind, they look like large women, not pregnant per se, but large.
some women can retain an hourglass-like figure up to delivery, so that from behind, you wouldn't imagine that a woman is in the last stages of pregnancy because she looks rather slender and proportioned from the back.
hopefully that can clarify what those remarks mean.
and since in most places, people still attach negative perceptions with being large or overweight, most women don't desire to look large or fat, pregnant or not. so when a person tells you that you don't look pregnant from behind and they got a big surprise when your turned around and they saw your belly, that's a [weak] compliment. i say weak because i think it's inappropriate to comment on anyone's body when you aren't familiar with that person and don't know how they'll feel about your remarks.
having said that, all your bellies are precious and i congratulate you all and wish you comfortable deliveries!
"Dogs stink too, but I like dog stink." ~ rileyb