Which lyme carrying species are you comparing? For Ixodes scapularis compared to Ixodes ricinus I see adult female sizes of 2.5 mm vs 3.6 mm. Not a huge difference. Are you sure you're not comparing larvae or nymphs to adults or even gorged adults?
I've taken a tick out of my dog in Germany and I've taken a tick out of my dog in Canada. The German tick was nearly an inch long with. The Canadian ticks I take out of my dogs are specks with legs, or at the most 2-3cm long.
