Regarding the overly large leg FAST packs and the need to bend the intake joint to attach them on the Yamato YF-19, basically it is a compromise between trying to get the FAST packs to look good in both fighter and battroid modes.
It again comes down to the extreme amount of anime magic used by Kawamori in the YF-19 design. The 19 & 21/22 use more anime magic in their transformation than probably any of Kawamori's other VF designs, with many parts either shrinking, getting bigger or completely changing shape between modes.
If you look at the attached battroid mode lineart below, you can see that Kawamori has drawn the leg FAST packs covering the whole of the lower leg, right up to the tail fins. Unfortunately, this is not at all compatible with the fighter mode, as the wings bisect the lower leg horizontally in the upper 1/4 to 1/3 of the leg.
Yes, Yamato could do a slimmer FAST pack that sit completely below the wing and keep the leg straight, but then in battroid mode, the FAST back would look odd as they would only partially cover the leg, leaving a large part of the lower leg bare.
Like I said, compromise.
IMO the best solution would be to have separate battroid and fighter mode FAST packs, but it ain't gonna happen.
Graham