Apple will only install a new hard drive with the same size of the one you are going to replace. If you want to install a bigger hard drive, you have to do it yourself.
If your Mac isn't in warranty, you may try to replace the hard drive yourself following iFixit steps > http://www.ifixit.com/Guide/MacBook+Pro+15-Inch+Unibody+Mid+2010+Hard+Drive+Repl acement/3030/1 Also, you can buy the hard drive on iFixit site
Visit thr OWC website. They not only have a large selection of compatible HDDs to choose from but also videos on how to install them. You will find that it is a very easy task. You will need two tools: #00 Phillips and #6 Torx drivers, both of which can be purchased in any good hardware store.
I upgraded my Early 2008 MBP 15" with a 750GB HD and 6GBs of memory bought from Other World Computing (OWC) located at macsales.com . It cost me about $250 for the HD, and external enclosure for the old disk, and memory. If you buy the disk and memory from them, you'll have to get it installed yourself though. They have videos online for your machine instructing you how to peform this task. This is not too hard if you have some basic skills... and of course the correct screwdrivers. They will sell you those inexpensively.. I think I paid about $5 for them.
Alternatively you can take it to an aftermarker repair shop.
Apple will be the most expensive b/c they will charge you list price for the HD and memory; and then add the labor on top os it.