What an interesting topic!
I'm not exactly sure what Wilder is referring to in his letter -- Robert Helpmann was known as something of a clothes horse, but it sounds like he's objecting to his stage costumes. Originally jodphurs were quite loose through the thighs (making it easier to ride astride) but they've become tight cut since the advent of stretch fabrics.
(and the frock coat that Wilder wore in the film was often paired with very tight trousers, so there's another element involved)
Interestingly, it's not easy to find a full-length photo of Wilder in the film but here he is
in the beige trousers.
Googling around, I found a charming description of Helpmann from a 1970 issue of The Age, an Australian paper. He and Nureyev were there to stage Don Q for the Australian Ballet, and this comes from a press conference.
"Sir Robert Helpmann, dressed as only Sir Robert Helpmann can dress. A magenta shirt and matching scarf. A suede safari jacket and tight trousers tucked into a pair of twinkling python skin boots. Silver gray nail polish to match his carefully arranged silver gray hair."
The title of the article was "Enter: two stars in gay clothes." The author goes on to describe Nureyev, and transcribe a fairly silly Q/A. It's here
if you want to look.