Mounted bull rush -- sizes affected?

[Tamago]

The description for a Bull Rush Combat Maneuver states:

prd wrote:

You can only bull rush an opponent who is no more than on size category larger than you.

Is this restriction affected if I am doing a bull rush while mounted as part of a charge? For example, if I am a Medium Human riding a Large Horse, would I be able to bull rush a Huge creature (since my mount is Large and we are Bull Rushing together), or only a Large creature (since my character is only Medium size)?

Thanks!

[Talonhawke]

It's either you or your mount Bull Rushing so if its your modifiers being used its your size and if its the horses it its size.