Well if we assume that a 10% increase in rider weight needs a 10% increase in planing area and a 10% increase in volume, and that you are starting with a 6 foot board, then scaling up the board for your weight could be done several ways ( the figure are only approximations ) :
1) Widen the board by 2 inches, no increase in length or thickness needed.
2) Widen the board by an inch and lengthen it by 3 inches, no increase in thickness required ( this keeps about the same board proportion as you started with )
3) Lengthen the board by 6 inches, no increase in thickness required.
If your aim is to keep turning well in small waves then (1) or (2) will be the best bet.
Often people try to keep the same length and width and rely on making the board thicker for added buoyancy when adjusting for a heavier person, but it's not a very good idea because it reduces the available planing area per pound of bodyweight. Adding volume addresses the paddling issue but does nothing for the planing area issue. Best to increase the planing area, which also has the happy consequence of increasing volume and thus buoyancy without making the board thicker.
One change which you might experience is due to the exercises done to increase muscle, they tend to be low rep high weight exercises which might reduce your finesse slightly and increase power. Tom Carroll said that he noticed that body building tended to do that. I'm assuming that you are not planning to add 10 kg of fat. I have added a planned 12 kg of bodyweight myself over the last 2 years from 74 to 85kg so have been thinking about the subject quite a bit ( it's muscle and bone not fat btw

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