Which release of Ubuntu are you using? If you don’t know, run
$ cat /etc/ubuntu_version
Did you install Java through the distribution, i.e.,
$ sudo apt install default-jre
That should automatically install a newer JRE (see also the installation instructions).
As you are running Ubuntu, you can also use the provided .deb package. Simply download the package, and then run
$ sudo install ./vassal_3.7.24-1_all.deb
and it will take care of downloading and installating needed third-party packages, as well as set up the desktop-integration (see also the installation instructions).
Vassal on Linux (and MacOS) will use the system look’n’feel (see also How to set the VASSAL Look’n’Feel). On Linux, this is GTK+, which will then use the current GTK theme. The spacing between (or rather margin of) elements is dictated by the used GTK theme. If you use the default GTK theme (Adwaita), then the margins around pictures on buttons, for example, is relatively big. One open is to change the theme used by GTK to a more compact one.
Remember to mark the post that solved your issue. That helps others find the solution to their problems. Only you, as OP, can do that.
Yours,
Christian