Is it based on monetary cost, filesize, perceived value or something else entirety?
Let's look at the definitions:
microtransaction: a very small financial transaction conducted online.
DLC: additional content created for a released video game.
Expansion pack: a separate product that builds on top of an existing game to add significant new features. (usually disc based)
Basically, microtransactions don't make the games bigger, they simply allow you to buy virtual items already in it.
For stuff that actually makes the game bigger, if it's downloadable online, then it's a DLC, if it's available as a physical copy, then it's an expansion pack. (DLCs are basically expansion packs, this just states the narrowest possible term you can use for them)
Micro transactions let you get new things like health kits, gun repairs,etc. Stuff you already can get in game but just want to pay money for to get quicker.
DLCs and expansions though give you new game play. A whole new weapon, a whole new area of the game map, or a new type of game play not available in the original game.