squbti, 25x25x1.5 is actually exactly what's specified. Turns out the sheet metal gauge is a bit different from wire gauge. 16 gauge sheet is 1.52 mm:
http://en.wikipedia.org/wiki/Sheet_metal#Gauge Note that the tolerance range is 1.39 through 1.64 mm.
Having said that, did you measure the thickness of your tubes? Mine are somewhere around 1.7, even though nominally it's 1.5.