Unfortunately Maven's internal support for report aggregation is rather poor and does have a number of limitations.
This may be addressed in the future by a module like dashboard to get an aggregate report.
When running findbugs on a project, the default heap size might not be enough to complete the build. For now there is no way to fork findbugs and run with its own memory requirements, but the following system variable will allow you to do so for Maven:
You can also use the fork option which will for a new JVM. You then use the maxHeap option to control the heap size.