The disk space required for the thin provisioned virtual disk depends on the demands of the guest OS. Each time the guest OS writes to a previously unused block on its partition, ESXi will allocate the disk space, i.e. increase the size of the thin virtual disk.
Windows - as most other dedup solutions - doesn't perform inline deduplication, but writes all new data un-deduplicated to the disk, and runs a schedule to optimize the disk usage hourly. In addition to this optimization task, there are other tasks (Scrubbing and GarbageCollection) which only run weekly. (for details see http://technet.microsoft.com/en-us/library/hh831434.aspx) Due to this post processing, Windows will - at least temporarily - require more disk space to save data prior to optimizing disk usage, which causes the thin provisioned virtual disk to grow.
You should alo be aware that in case you need to recover data from a backup (in case of a guest file backup) to a dedup enabled partition, you may - due to the post processing dedup - run out of disk space if the amount of backed up data is larger than the partition size.
André