Is the problem being faced during designing of the test cases or during the execution of the tests?
If the problem is of during the execution phase, then are you running the tests using UI? If yes, then you may consider running the tests command line utility testrunner.bat/sh of SOAPUI_HOME/bin.
Another aspect to look at reducing is that - make the tests modular and use data driven approach. Usually what happens is there might be minimal data changes and tests are being cloned which is one of the reason that contributes huge project size.
Keep only the relevant suites in a project, instead of just keep adding all the suites. If you think some the suites are not really needed to be together, just split them into different project.
If the properties are to be shared when you split the projects, use properties files and remove those properties from the project (mostly I am taking about project properties), that will also reduces the project file size. Whenever you need, just import. Of course, if you are executing tests from testrunner, you can even pass it using System argument without manually importing those properties, soapui dynamically loads them run time.
There can be more, but again context is more important to be more specific.
Hope these helps.
