I felt the same until I tried it. It feels very skinny and unobtrusive in the hand, it weighs less, and it seems to me to be ergonomically superior. I struggle to use the old RIS style now, it just feels like I'm tring to manipulate the gun whilst its attached to a clunky cheese grater.
Whilst the mk18 had certain real steel advantages due to heat dissipation, these are not relevant in airsoft. I really like Magpul as an innovative modern business, and their R&D on M Lok was superb. I remember getting one of the early MOE handguards for my PTW about 10 years ago and liking it's 'high speed, low drag' ergonomics, and what Geissele have done with that concept is refine it further. HAO have done an excellent job again at making it affordable for airsoft. Firm handshakes all round.