Despite descriptions they're both multi-part kits, made completely of resin. The Leviathan was "okay" to build. It's large, chunky and has a decent enough surface area to glue (used super-glue gel and some 5-minute epoxy for more load bearing parts like the balls on the feet).
The Leviathan is (mass-wise) 2-3 times the size of a normal dreadnought, but about double its height. It's still not as tall as an Eldar wraithlord.
This is off the ole' google.
The Leviathan is bulky as all heck. Front to back its also quite thick.