Again.. this boils down to a simple matter of $$$.. design of a larger seat.. tooling for making them.. making them.. and then once you place them on a train, will they be safe for a smaller rider if they are needed.
Seats for coasters are generally designed for slightly larger than average people, and it tends to fit about 80%-90% of the population. In the end, remember all these companies, the parks, the ride companies, are in this to make money and they have to look at their bottom line in costs.
B&M does make some larger seats for their stand-ups and inverts bt it's up to each park as to if they will order some. Again is a smaller guest going to be safe in such a seat? if it allows too much room, a park will generally opt not to get the seats for sheer capacity reasons which then inpack profits As much as we like to think they do this because they love us, it's a bussiness and they are in it for the money.
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MF 2000 - 269 laps
MF 2001 - 59 laps as of (5/28/01)