Since the Viking Age of around 800 C.E., cats have actually grown in size, which is the opposite of what is seen in most animals as they become domesticated.
Via Sciencemag.org, a report in the Danish Journal of Archaeology has shown that domesticated cats have grown by about 16% between the Viking Age and today.
For comparison, the average domesticated dog is around 25% smaller than a wild gray wolf.
While this study, which looked at cat bones from over 2000 years, only focused on Danish cats, there was also a study in 1987 from Germany that also reinforced the idea that medieval age cats were smaller than those found today.
Source: IGN.com Skeletons Reveal Cats Have Grown Larger Since the Viking Age