There would need to be a little more detail involved.
For instance:
6' man of average build
Age 18: 230#, kind of plump
Goes on a health kick
Age 24: 180#, In trim shape
Goes heavy into weightlifting and martial arts.
Age 30: 230#, Pumped up with muscle shape
That age 30 weight is not grossly muscled, you can be 6' and 230# and still have no restraint on movement. (The above describes a friend of mine). . .perhaps lifestyle changes would allow one to do the slim/normal/stocky transition and change your base?
Then again, I'd say that my friend's transition from 24 to 30 represents a large increase in in temp strength, and therefore bonus. . .
So perhaps the "Very Muscular" weight gain is to a great degree self regulating, you gain 10# of muscle, but you also may gain +5 St bonus which per the encumberence rules may just mean a net 0 or perhaps more carry weight allowed.
In real life, endurence runners are all stringy muscle, so the logic of this would probably lean toward the "self regulating" angle. . . .then again, RM doesn't really allow you to just decide to pump iron until you get to 100 strength. (Sometimes the "Potential" mechanic conflicts with reality.)