Says who? Why, do you suppose, would Skynet make a bunch of T-700s and never skin them? Oh, right, cause Cameron's Skynet didn't have odd integered series Terminators, only 600, 800, and 1000. McG can suck it for not paying more attention to details.
The fact that T-700's are foot soldiers not infiltraitors makes there need of a skin covering redundant. And T-700's are just to bridge the gap between the T-600 and T-800. Think about it T-100's, who are T-1's basically!, are a 100 series. If the go up in model the way you say it would go: T-100, T-300, T-500, T-700 and T-900. So I can see your point, but your right! This is'nt Reese from 1984's future rant. It's a diffrent one because of the events at the end of T2 so it effects the time line. Altering models and there size! Maybe because Skynet had no T-800 arm or chip, from T1, the T-600 template was altered. Not enough info to build them advanced enough.