How can it tell you anything? The midwife doesn't know what you looked like pre-pregnancy!
I've got a very rounded stomach that you could measure as a bump if you were so inclined, but at 19 weeks I know that it's not that much bigger than normal - how would the midwife be expected to know that though? It's just where I've always carried my weight... (I'm 5'10 and size 14 with slim arms and legs).
She's feeling the top of your womb (fundus) and measuring the distance from the fundus to your pubic bone. If you are very obese or very very underweight it might affect the measurement, which is why generally you're offered more scans. But generally when we lie down our fat levels out and the vertical measurement not affected by usual levels of abdominal fat. She couldn't have measured a bumper pregnancy because you didn't have a fundus!
It's not the bump we measure but the height of the fundus to the pubic bone. This normally correlates with how many weeks pregnant you are. An overweight woman would measure the same as a thin woman of the same gestation despite how much excess skin/fat is there. Of course this can vary slightly but you get the jist :-)