memcached is a simple thing: it is a dictionary (like in a php array or dict in python) a limited amount of hanging in the memory of a single process with a simple access Protocol that can be used by other processes.
If a record total of more than exhibited a limit on the size, the old data (which is the longest not used) will be overwritten.
Think of it roughly like $a['vasia'] = 'petya', but like data in the $a variable is preserved between calls, available not only for the specific script + security of these data is not guaranteed.
Regarding limitations on the size of the values should read here
You at this stage want to record tens of thousands of large objects (by the way, I do not understand any of the objects — that is 6M text, something about youtube) in memory to provide fast access to them. Don't know if this is the problem you want to solve. Then you need to recompile memcached and a large amount of RAM.