I don't have a scale with me but Lenovo lists the T460s at 1.4kg and the X1 at 1.13kg. So it's significant (~20%) but not that high. Note that the T460s is a different model to the normal T460.

Oh, I missed the "s" part and looked up the specs of the regular T460.

And it is MUCH thicker (or at least it seems so side-by-side)..

Which it has to be to have space for the ginourmous rj45 plug... (I can easily live with the small dongle to get native rj45 on the x1s)

That's my pet peeve with my current (1st gen) X1 Carbon. I hate having to carry at least 3 dongles with me nearly everywhere. I usually travel with an mDP to VGA, mDP to HDMI, and a USB3 Ethernet adapter. I'd much rather have something like the T460s with two of those built in.

The specs list it as 1 to 3mm thicker which doesn't seem like much.

This is because one (the X1) is tapered - you "experience" the thin side (keyboard) while the thick part is on the back.

The 450/60/70 is just a flat slab so it maintains its thickness along its whole surface (although the 460 and 70 cheat and they become thinner about 1 inch before the end, so they look a bit thinner when looked at from the front).

I have a 450 and don't mind it at all.

The T4[567]0s series are tapered at the edge as well. The -s suffix is the lighter version, basically similar to X1 series but with more ports/battery options/upgradability.