They don't? I guess the EM4/Copplestone guys are smaller?
yep em4 = old copplestone and new copplestone are quite similar, although definitely more realistically proportioned than the old cadians, which are quite bulky.
However, the general appearance is quite similar, so I use them in the same army, but not in the same unit.
I like the old cadians myself, but could not come up with anything that really resembles them.
The starship trooper colonial marines are also a close match, and I tried a bit conversion on the plastic cadians, notably the helmet, which then got quite close in the general style.
only chance is to see the body armour as "old style" and accept the rest as "new style".
what You could also do is head swap the whole range with the old catachans - the general appearance is thinned down, but You instantly quadruple Your range
true catachans
true cadians
cadians with cat. heads
catachans with cad. heads
boots, pants, gear, weapons are the same
only difference is body armour/jacket and helmet, but mixed
what You get is a "vietnam style" IG veteran army