Hi, I can only share what I knew from my sister's boy (he is now 17 years old already) with you and from here you probably would be able to access your baby in his development.
I recall that my nephew has a bigger head (even now he is a big head.....LOL) than his body and when he had reached the time to start crawling, he banged his head constantly on the floor/mattress whenever we put him on his tummy. At first, we thought he was just lazy but realise that he was actually struggling to balance his body. After some observation, we noticed that it was his head that's giving him the problem. I suppose this can also apply to baby who is overweight.
Then few weeks later or maybe a month or so (cant remember since it was so many years ago!), he started to move his knees and raised his butt up and finally, he sit upright. In my memory, he barely crawl because of his heavy head! Hence, all I can say is NOT all babies will crawl (my mother can confirm this). If babies realize that they have to struggle to do something, they have this basic instinct to move on to the next stage/phase (e.g. crawl, walk etc) pretty quickly.
If you are still worried, you might want to schedule an appointment with the pediatrician to get his/her opinion.