So Using the 1st level ability of Mammoth rider, could a medium character who qualifies for Mammoth Rider change his steed to one he would normally be unable to have such as a medium animal?
Steed (Ex)
A mammoth rider can choose from among the following animal companions to serve as her loyal steed:
Arsinoitherium, aurochs, baluchitherium, camel, cat (big), elk, horse, mastodon (elephant), megaloceros, rhinoceros, triceratops, wolf, or woolly mammoth (use stats for a mastodon).
A GM might expand this list to include other possible steeds. This steed functions as a druid's animal companion, replacing any animal companion or mount gained from another class. The mammoth rider's class levels stack with other classes that grant an animal companion to determine her effective druid level.
Gigantic Steed (Ex)
A mammoth rider's steed increases to Huge size. The creature receives a –1 penalty on attack rolls and to AC, and a –2 penalty to Dexterity (to a minimum of 1); its base damage increases by one size category; and its reach increases to 10 feet. It also gains a +2 size bonus to its Strength and Constitution. At 3rd level and every two levels thereafter, the Strength bonus increases by an additional +2; the bonus to Constitution increases by an additional +2 at 5th and again at 9th level.
Like a size Huge wolf? Sounds reasonable to me.