Argh, misread the question.
Yes, the steed still increases as written in the Animal Companion table, plus what is in the prestige class.
The mammoth rider’s class levels stack with other classes that grant an animal companion to determine her effective druid level.
So, the basic stats for your Animal Companion are determined by:your levels in the classes that granted the AC+your levels in Mammoth Rider.
Without that, it would have too few HDs to be useful.
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Animal companions have specific rules for what happens when they increase in size, rules that supersede the normal rules for size increase.
So your Animal Companion will become Huge if it isn't already that but get nothing for the size increase.
Instead, it gets a size penalty to attack, AC, and Dex once and a size bonus to Str and Con at levels 1, 3, 5, 7, and 9.
Personally, I find it strange that the steeds that are already huge will get a further penalty to attack, AC and Dex, and that those that aren't huge to start with will not benefit from the size increase (from my POW, the size bonuses to Str and Con are the benefit from being an Animal Companion, not for the size increase) and probably will apply the rules for increasing the size of monsters found in the Bestiary, but that is a GM houserule, not the RAW.