The default for Lens Sharpness Optimization has changed in PL9 together with its value range. Default for LSO in PL9 is 100 which corresponds to PL8 0.0 setting. The default for PL8 is +1.0, which corresponds to about 133 value in PL9. This may be one of the reasons for the difference you see. See Lens Sharpness Optimization V2 question - #3 by Wlodek for the mapping.
Typically, I get slightly smaller jpegs in PL9 than in PL8. Didn’t look at it closer, since I don’t see any significant differences between PL8 and PL9 jpeg exports.
No samples, so speculating only. You may compare DQT and DHT tables in both jpg files and tell us your finding.