Try to set:

$HistoryLength = 1 (or 0)

rather than clearing Out manually. But do so at the start of your session. This will only store one 'Out' not more...

Basically, every time you input something and run it, it will be stored in Out[xyz] where xyz is a number. By default, $HistoryLength = infinity, such that everything that is outputted is stored inside the memory, gradually building up over time. By setting HistoryLength to some low number, it will only store the last xyz number of outputs...

The example will depend on what you had for input and can vary orders of magnitudes...