Shared account can have unlimited space as there is only one user adding content but reseller account must set limit to the amount of space allotted to the users, as there can be multiple users in an account adding content and creating files and folders and so the space on a reseller account cannot be unlimited.

The main reason for space difference between Shared and Reseller accounts lies in inode limit. An inode is a data structure that keeps a tab on the number of files and folders in an account. If the inode count is more, then more resources are utilized and the higher will be the file system overhead.