In general, the sizes for a normal rbf file and a file for partial reconfiguration differ due to this:
Configuration bitstreams used in a non-PR flow are different than those used in a PR
flow. In addition to standard data and CRC check bits, configuration bitstreams for
partial reconfiguration also include instructions that direct the PR control block to
process the data for partial reconfiguration.
Further it also depends on the different modes you choose.
The SCRUB mode you're thinking of refers to the old Stratix V flow, not your device.
In general, for Arria 10, the final bitstream file size and time to reconfigure your design is proportional to PR region height. The easiest thing to do to shrink .rbf file size is to make your PR regions shorter in Y and wider in X. This gets into how the device masks off SRAM bits to not reconfigure parts of the design and the way CRAM bits (configuration RAM) are set.