Hi,
Do you mean that you have only Full GC(major collection) going on and their are no minor collection happening ?
If Yes , then if the Pauses caused by major collection is bearable then you shouldnt worry too much about it and should let JVM do its work. If you are getting very long pause and its affecting overall performance then I think before changes any defauts collectors/parameters you should closely monitor your system, verbose logs, different conditions in which you see this behavior.
Also check if you are not calling any System.GC() is your java code as this call would do a major collection and generally its not a good idea to call system.gc(correct me if I am wrong)
http://java.sun.com/docs/hotspot/gc5.0/gc_tuning_5.html explains the working of GC with different options.
Jiten