Firstly, the jpeg size will be smaller anyway if there are large areas of plain colour in the photo. I've just had a quick glance at some of my EM5 wildflower shots which have varying levels of detail in them and the jpegs vary between about 6 and 11.5 megs.
I have the superfine jpeg set together with raw. Olympus customising can be a bit daunting to start with and having chosen the largest jpeg you then have to go into the custom menu to get the superfine setting instead of just 'fine.' Go to custom (cog) 'G' which is the 'set' symbol/color/WB - then scroll down to the last but one option 'Set' and from there go into set 1, where you can alter the settings similar to changing the time/date - the 1st box should be 'L' for large pixel count then the second box is the one you need to move the arrows up or down to select 'SF' then press OK and it should be what you need!
Confirm it be checking the viewfinder/rear screen which should show L SF+RAW in the appropriate position.