Skyrim is heavily single threaded, basically it uses just 1 core, any spillover going to a 2nd core or at most a 3rd. In games such as this, core speed is going to show more, so the higher core speed of the g3258 (I'm guessing it's got a decent oc) is what's making the difference. The engine skyrim (and fallout) uses is all about the cpu, very little to do with gpu when compared to other engines.
I'm running 130+ mods, most of which are 4k textures, town, fauna & flora, etc, an enb for weather on a 3570k @ 4.3GHz with a gtx660ti. Maxed out everything I see 64% gpu usage when staring at high grass and 54% cpu usage on 1 core. I don't get spillover. Constant 59-60 fps, no matter what.
Even with 3.7Ghz vrs 2.7Ghz of the old processor, the only thing that's changed is the enb is now smoother. Enb's are high cpu usage and the settings, textures etc have basically capped out on that 750ti. You'll only see raised fps now by lowering things such as sight distances, and especially shadows. Your new cpu has the speed to raise the fps but is now being limited by the gpu.