I don't have any of the War of the Triple Alliance figures (as least not yet, we'll see how long my manly resolve lasts...) but based on comments from a reliable someone who purchased them, it would appear the WTA miniatures are taller and leaner than the old Foundry ACW sculpts and even some earlier Perry ranges. Alan Perry's sculpting style has evolved over time and his figures have lost a bit of bulk and gained a couple of millimeters in the process. So expect the old Foundry range to look short and stocky next to the more recent Perry stuff, with noticeably larger heads as well.
Mind you, there are occasional size inconsistencies within some Perry ranges as well : in the ACW range, the dismounted Union cavalry troopers (released some ten to twelve years ago) look a bit small next to their intended command pack (sculpted circa 2015 or 2016 and containing noticeably taller miniatures).