Why is the "empty" pLOC vector sequence larger than the pLOC-RFP control?

The pLOC ""empty vector"" has a stuffer sequence that is 1. 5 kb. It contains chloramphenicol resistance and a ccdB negative selection marker. Both are removed and replaced by the ORF in the expression clones. The RFP control ORF is about 0. 7 kb so there is about a 0. 8 kb difference in size.