The couple 18" NECA's I've ever owned were about equal to their 7" counterparts in the paint department, the only quality upgrade you get with the bigger size is better sculpt appearance, more detail,.
Here i found a good example. the same figure in 7" and 18" size:
https://www.youtube.com/watch?v=XKUV_bKhciU&feature=fvw
https://www.youtube.com/watch?v=ty6rNiVNc10
...the 7" looks terrrible, the 18" outstanding have this guy in my collection.