|You are getting confused between system memory (RAM) and hard disk space.|
The only similarity between the two is that the size of both is expressed in the number of bytes they contain, either MBs or GBs or even kBs if it is small..
The space on the hard disk allocated to the Recycle Bin can be a adjusted. Right click on the recycle bin and select properties. You can then adjust the space allocated to the recycle bin. Once it is full, the system starts deleting older files but for most purposes the default is adequate,
In reality the files stored in the Recycle Bin are little different than all the other programme and data files stored on the hard disk. They are just stored in a special folder where they system know where to find them.
As for system memory (RAM) allocation, that is best left for the system to manage. It is far to complicated for mere mortals to understand. That is best left to the Gods of System Programming.