Over the years I have known a few people that use Mini stallions for the purpose of teasing their big mares.
Unfortunately, Gage, yes, many times these people do actually run the Mini out on pasture with the big mares. The idea is, he's out there with them 24/7 so when a mare comes into heat it's going to be noticed right away. Convenient, because even though the stallion is right out there with the mares, he's too small to be able to get any of them bred. The owner can go and get the mare & breed her to the full size stallion she's meant to be bred to.
I don't think much of this practice because
1) it's much too easy for the little stallion to get injured by one of the big mares
2) many many times these breeders have commented on how ill tempered the little teasing stallion got. Sometimes the little guy gets a mare or two of his own to breed, but many times it seems these breeders just figure it's amusing that this little stallion is out there with all these mares but never, ever "gets any". The stallion is in a perpetual state of frustration.