When people call out microaggressions, a common dismissal I read or hear is “if they’re so small why are you treating them like some huge deal?”
Think of microaggressions like termites. One termite won’t destroy a house. Thousands of termites eating away at the foundation, bit by bit, ultimately make it collapse.
Seeing one microaggression in isolation makes complaining about them petty. Knowing the context of how they damage people makes calling them out important. Knowing this context and still using the “it’s just one small thing” dismissal makes you an asshole.
If you’re not convinced, see this comic illustrating how the damage is done.
Better yet, read a (likely incomplete) documentation of the microaggressions one woman deals with online.