Size tag with Coordinates does not work when summoning Slimes and Magma Cubes

I tried summoning a Slime and a Magma cube using the following command:

/summon Slime ~ ~1 ~ {Size:[16]}

This summons a slime of the smallest size. But when using the following command:

/summon Slime {Size:[16]}

It summons a size 16 Slime. The same goes for the Magma Cubes.