Do wholesale tiers work per variant on a product with sizes or colours?

Tiers are set per product, so every variant of that product uses the same tier table. You cannot set different tiers for different sizes.

The quantity that is checked is the quantity of each cart line. Each size the shopper picks becomes its own line, so a buyer who takes 4 of each of five sizes has five lines of 4 units. A tier of 10+ would not apply to any of them.

If you want mixed-size orders to qualify, set your tier thresholds with that in mind, for example a lower Min quantity.