I've fractured the growthplate in each ankle before but there was almost no swelling. Why could this be?

It is strange? When you have fracture in the ankle this happened due to trauma, I am very surprised you no swelling.
Sprain . There are 4 different growth plate injuries that can happen to the growth plate. Stage 1 is the least severe which is almost similar to a sprain up to stage 4 which is a surgical emergency. So depending on the type of injury you have suffered can affect the amount of swelling you may have.