It can affect those won (ahem) "sprinter's thighs", I had the same with an Archie's grobags saddle pack. I think taping something to the back of your Seatpost (bit of broomstick/Seatpost offcut/part of a lock bracket) could help space it back a bit. Get creative.
Anyone experienced some leg rub (Back of thighs) when using the Restrap saddle bag holster? Got about 10L in it at the moment.
I have quite a lot of set back and my saddle (SMP Forma) is setup as far back as it goes.
Any ideas / help will be much appreciated