There is not any penalty for doing smaller additions, and no maintenance costs for a larger addition.
So the risk of a too small arena is that you are missing out on some ticket sales that you could had with a larger arena. It might also lead you to set too high of prices for tickets. Let's say that you have 1000 seats and they are always full, so you raise the price to $12 and they are still full. But it might be that you could sell 1500 seats at $10, so you are actually still missing out on $3000 in revenue (numbers just made up for illustration of the point).
The risk of a too large arena is that you are spending money on an arena that might be better spent on a new center or coach (trainer). You might also lower your ticket prices in an effort to fill the seats, when it might be better to have empty seats with a higher ticket price. So for example if you had 5000 seats that you dropped the price to $5 to fill them, you might be better of with 4000 fans paying $7 along with 1000 empty seats.
There is another advantage to smaller builds. They are finished quicker. You can't use any new seats until the construction is completed. So you might be able to get a small addition ready for your next home game, and then another addition for a later home game. But if you did a single larger addition, you might not have it ready for the first game.