The maximum amount of memory used by Java is specified on command line (max heap size parameter -Xmx). If nothing is specified a default value is used. This default value has increased within the last year. Before it was about 200 MB and nowadays it is much higher.
Therefore I assume you are using a very old Java 32 bit version which therefore used the old default value.
You can specify the values for Eclipse in the eclipse.ini and therefore set the amount of memory you want.
See also FAQ How do I increase the heap size available to Eclipse?. Lowering the heap size is the same process.
^But I have to mention that in general 64bit processes require more memory because of the larger address pointers they use. Therefore you may get OutOfMemory errors when trying to run Eclipse with a max heap size of only 200 MB.
Eclipse was and is a huge memory monster. The more it gets the better it runs...