There IS one way of increasing Planet/System sizes!
The Planet mod!
Or the German version:
That's a link to the download for the Planet mod, from the parent site of this one, armadafleetcommand.com.
If it tells you to log in, try using the same login details that you use for this site. They <should> work.
If you can't direct access the download(s), the english version mod is second in the list on the following page, whilst the German one is third:
The mod increases the maximum population of the planets by 2.72 times, as well as making the Homesystems far larger.
This means that ALL class M planets (Irrelevant of size) have a population of 255, and (As an example) Jupiter and Neptune are now Class M planets - Sol goes from holding the 300-odd population it has, to having 910 population.
This is the only way that I know of to increase the size of your systems, other than by terraforming.