Our body size measurements indicated a homogenous native population from Russia (average body size ± standard deviation = 4.72 ± 0.59 mm), Italy (4.82 ± 0.51 mm) and the Iberian Peninsula (4.62 ± 0.41 mm), as well as two isolated expanding ones in the Baltic states (4.06 ± 0.47 mm) and northeastern Germany (4.42 ± 0.50 mm) (Fig. 3A). Body size, as measured by the prosoma width, of native populations was not significantly different, while the expanding populations were each distinguished by a significantly smaller body size from the native populations (anova, Games–Howell post hoc test, P < 0.05). In addition, we found a significant size difference between the two expanding populations, with specimens from northeastern Germany being larger than those from the Baltic. Overall, the body size data were highly congruent with our microsatellite analysis.