Most (not all) of the strategy around enlarge and reduce is around attacks that can't be used on someone of a certain size.

For example, take bash.

You cannot bash someone who is two sizes smaller than you (e.g., normal size giant trying to bash normal size gnome.)

On the other hand, being one size larger is advantageous for bashing.

Therefore, if (defensively) you're worried about being bashed, reducing to be two sizes smaller than your opponent is ideal, if possible, such as a human reducing to fight a giant. On the other hand, if you can't possibly be two sizes smaller, such as a human fighting another human, it's probably to your advantage to be as large as possible.

Another example would be blackjack, which can't be used on someone two sizes larger than you.

In most, but not all cases, other aspects of advantages for reducing/enlarging beyond what special attacks are advantageous or possible are kind of a wash. You pick up an advantage in one area but lose one in another area and it all roughly evens out.