What happens when you use a specific CRF value on a already encoded x264 video?
When decoding a file, the decoder attempts to recreate the source material with the information provided in the file. But it can not be a 100 percent accurate representation, so the decoder creates new pixels that are estimates of what was included in the source 

When you encode for the second time with CRF 17, the encoder does not know these new pixels were generated by the decoder and were not necessarily faithful to the original material. But the encoder still does its job, and attempts to encode these new values accurately. 

So it spends bits encoding pixels that were thrown away and recreated by the first encode, making the file larger.