I tested several sizes in arrows / javelins projectiles and depending on the size of the sprite they go faster or slower
is there any other method of making an arrow fired by an archer of going faster / slower without changing the collection?
yes, it's odd that the size of the projectile should have any effect at all (it really shouldn't!). the speed should only be determined by the speed specified in the monster's attacks and then by the acceleration from area of effect detonations (that don't destroy the projectile in question).