It simply isn't possible for VirtualBox to store more than the creation-specified amount in the VDI, so the error must lie elsewhere. Answers that spring to mind include user error, not understanding the difference between decimal and binary GB (*), general misuse of rounded numbers, and perhaps a guest filesystem with sparse file support and a lot of mostly-zero files.
<rant> (*) For example, 450.41GB (binary) == 483.62*10^9 bytes. This is because 1GB = 1024^3, not 1000^3. Computers do not have 10 fingers and hence have no special affinity for base 10. However, some people insist on muddying the waters by inventing decimal variants of GB, MB etc, then using it where it really isn't wanted at all. For example, in the screenshot to the above right. Then these same ignoramuses have the gall to claim that the old terms are confusing because people abuse them, and the solution is new and more verbose terms such as GiB... which of course will be abused by a future generation of ignoramuses in exactly the same way. The actual solution is to use the original terms correctly and consistently throughout, (as VirtualBox does).
</rant>
