Stout or not, Augusta Victoria was a pretty active rider - she once broke a leg jumping.
Apart from Queen Mary, the only other non-rider I know of was the Empress Alexandra. She disliked riding intensely and only got on a horse when she had to. I have a picture of her on horseback in the unform of her uhlan regiment and she looks quite spectacularly miserable.
Ann
Early on, at least, she looked rather sharp in her white uniform. VL followed in her grandmother Vicky's footstaps with the Death Head Hussars, I believe. I think the photo of Vicky during her review shows the same uniform that VL would be wearing during her own reviews.
Yes, AF did look rather uncomfortable on horseback. I don't know her feelings about it though. I never read anything of QM being on horse nor seen any photos. The British royal family didn't seem to review the troops in the same fashion as some of the other royal families--at least the women. Princess Mary was a huge horsewoman as were her female relatives on her paternal side. I don't know whether QM just had an anathema towards it, a bad experience ala Princess Diana or why she seemed to avoid horseback.