Depends how inefficiently you're storing the data - you can _certainly_ incur overheads - for example, XML is around 10x the memory footprint of the file at rest.

But at least now we've moved on from blaming open - check what you're inserting into the hash. How many key/value pairs? Are you creating nested data structures? (hash of arrays, etc.)? Because all these things add up - you _can_ expect the memory required to be larger than the raw input.