New player here, what's to stop the large monster from shrinking and expanding its way across the board over and over in the same turn without spending any movement points after a move action? Or at least expand after each movement point is spent to double or triple the distance covered.

A monster is prohibited from shrinking unless it is spending a movement point (or otherwise moving). It is prohibited from expanding unless it:

-first declares a valid action or other ability which interrupts its movement

or

-is forcibly interrupted by another game effect

That is, monsters cannot shrink and expand at will- they have to have a good reason to do so.