Well you're right it's actually not thinner than my previous phone, they are both the same at 7.9mm . It did "feel" thinner in that it's easier to use one handed which I guess is due to the Pixel3 being close to half a centimeter narrower and the edges being more rounded.

Somehow I perceived the narrowness as being thiness. In terms of weight though the Pixel3 is 12 grams heavier.