I have a massive bump, and I'm only 15 weeks on my first.
I was never skinny, but I was a size 14 waist/hip pre-pregnancy... and slowly but surely on my way to becoming a 12... but since getting pregnant, my already rather ample bosom has taken on a life of its own (I was a 34FF, now I would say a 36H, possibly bigger) - I can't even squeeze closed my previously baggy shirts across the chest, there's a good 2 inch gap when pulled as tight as I can.
And then there is the belly - i've seen women smaller at term than I am now. If the doc wasn't certain I was only having one, I'd think there was triplets in there
I put it down to stopping any sort of strenuous exercise, and drinking more milk and fruit juice (when I used to drinkl only water). I used to do zumba for 45 minutes every single morning, but since getting pregnant the bouncing around felt wrong. So, I do very little exercise... apart from the occasional walk, which just doesn't seem to burn any calories.
But to be honest, I'm just going to accept whatever pregnancy throws at me and whatever shape comes with it. Once Le Baby is born, I can work to get back in shape... whatever it takes.