small player not working

[tame basin]

i am running the version of 1.21.5 i got skytils from modrinth, i tried to turn on small player buut it didnt work instead it just make my and the other poeples legs fast, i dot know if this is a problem with one of my mods (i show in the video) or a problem with the mod, or even because i got the mod from modrinth