So do you get excited by shared hosting plans that mention un-limited bandwidth and un-limited diskspace or thousands of GiB of diskspace with thousands of GiB of bandwidth ? There are practical limits to un-limited though. You can store theoretically store Terabytes of data but when would you finish uploading it though ? and it can’t all be served because sufficient number of storage IOPS(IO operations per second ) are simply not available on shared hosting plans, you are probably sharing a machine with several hundred others and eventually everyone’s site ends up slow because there is not sufficient CPU/disk IOPS and memory available to your site to utilize the supposedly huge bandwidth and diskspace allowance.
The VPS servers solve the problem of CPU/Memory availability by resource reservation, carving out a fractional machine for you with full OS Virtualization using Xen/KVM supported by VT compatible hardware.
VPS hosting providers also don’t resist using larger but slower disks advertising the large diskspace they provide but failing to mention if they disks used by them are spinning at 15K RPM or 10K RPM and how many spindles per server have they added.
The speed of your application is limited by the slowest components on your hardware (real or virtual) which happens to be disk-subsystem. Stop worrying about the size of diskspace and instead start asking about IOPS available to you instead.