you misunderstood. Your mother and father's weight have nothing to do with it (or very little at least).
Your genetic potential is up to how much testosterone you produce, basically. More testosterone you produce then the more muscle your body can support naturally (hypothetically if we magically gave you 100 more pounds of muscle right now with your test levels, it would go away rather quickly in the coming months because your test levels aren't up to that task of maintaining it).
Also you can look at your skeletal frame (thin, thick, etc) and determine how easy it is to build muscle. A thin framed person will have a harder time building all things equal.
Also, you can look at the insertion points of your muscle to determine your genetic potential.
Its a number of factors, but just looking at your mother/father is drastically missing the point and revealing a fundamental misunderstanding on your part of the relation between diet/musculature.