My X-wing measured 45 mm exactly (nose to end of the engines). The base measured 1 and 19/32nds long (just over 1.5 inches) and 1 and 9/16ths wide (again just over 1.5 inches).
I've got to imagine you were measuring the tops of the bases not the bottoms. The sides of the bases are sloped downwards. The bottoms of the bases are exactly 40mm square.
Here are some other measurements I made:
X-Wing, nose to engine: 45mm
TIE Fighter center of solar panels: 32mm
TIE Fighter bottom of solar panels: 22mm
Y-Wing laser tips to engine: 59mm
TIE Advanced center of solar panels: 33.5mm
TIE Advanced botom of solar panels: 25mm
X-Wing height : 18mm (0.8 in)
TIE Advanced height: 22mm (1 in)
Y-Wing height: 13mm (0.6 in)
Height measurements include the peg as they were concerning storage.
Those are WotC SSB minis on the black bases. Only a single altitude peg was used to bring heights closer. Larger images are available if you remove the "_md" from the image url but I wanted them to fit on the forum page.