At his age, it's unlikely to be anything apart from exploration and curiosity, and it seems within the scope of "normal" as long as he's not doing it to other women, or talking about women's breasts to a point where it seems odd.
It sounds like he's testing limits and isn't respecting boundaries by continuing to do it after you've asked him not to, which again, is pretty typical kid behavior. I'd focus on getting him to understand and respect boundaries rather than on the fact that he's touching breasts. He just happened to pick that to test his ability to exceed parental boundaries.
I agree that this is normal curiosity, but my concern is that boys don't usually do this when their dads tell them to cut it out.
A 7 year old isn't like a 4 year old, who might be curious and touch his mom's boobs.
Burnalbee, it's time for you to take control and tell your son to stop grabbing your wife's breasts.
I'm also a little curious about your comment on the size of your wife's breasts, is this a big deal or point of discussion, such as, do you tease your wife about them or does she accentuate them? I'm thinking that if I had a husband who liked to show off his whatever -- for example he had great thighs and liked to wear shorts, or had a cute tush and liked his pants tight -- and if it was talked of proudly or admiringly between us when our child was around, it wouldn't surprise me too much if the child picked up on it and would touch or comment on that body part of my husband's.
Where you able to control your son fascination with his mom breast ?