@SubscribeEvent
    public void onSizeChange(PlayerEvent.Size event) {
        if (!(event.getEntity() instanceof PlayerEntity)) return;
        EntitySize newSize;
        switch (event.getPose()) {
            case CROUCHING:
                newSize = EntitySize.fixed(0.6f * SizeMod.sizePercentage, 1.5f * SizeMod.sizePercentage);
                break;

            case SWIMMING:
            case FALL_FLYING:
                newSize = EntitySize.fixed(0.6f * SizeMod.sizePercentage, 0.6f * SizeMod.sizePercentage);
                break;

            case SLEEPING:
                newSize = EntitySize.fixed(0.2f * SizeMod.sizePercentage, 0.2f * SizeMod.sizePercentage);
                break;

            case DYING:
                newSize = EntitySize.fixed(0.01f, 0.01f);
                break;

            default:
                newSize = EntitySize.fixed(0.6f * SizeMod.sizePercentage, 1.8f * SizeMod.sizePercentage);
        }
        event.setNewSize(newSize);
        event.setNewEyeHeight(newSize.height * 0.9f);
    }

I can't seem to figure out what's going wrong with my code. It seems like it should only affect players but any mob that spawns has its hitbox changed as well as any item that's dropped. Does anyone happen to know what's going on here?

the Event dosen't exist in this class, it's in EntityEvent.Size, I think the Event was moved to the Entity class, because it affects all Entities

Ok, but I still have the line

if (!(event.getEntity() instanceof PlayerEntity)) return;

so do you know why it's not only affecting players?

use debugger and look what's wrong