Why does BitLocker need a minimum volume size of 64 MB?

Since the future of TrueCrypt appears to be still unclear, I figured I'd try to get my stuff migrated into BitLocker at least for the time being. I nearly never have to access my encrypted data from anything that's not BitLocker-capable, so cross-platform compatibility isn't a big deal to me at this time.

However, I am having a bit of an issue understanding the minimum requirement of a 64 MB volume. With TrueCrypt, I was able to protect small files (and most of my protected files are fairly small) in containers down to 300 KB or even less. When I finally created a VHD of an appropriate size last night (100 MB), it seemed the file system itself only took up about 3 MB and encrypting it with BitLocker didn't appear to take up any more.

While 3 MB is still an order of magnitude larger than the smallest volume I could make with TrueCrypt, it's still relatively reasonable in comparison to 64 MB. This is an especially large amount of overhead (and largely wasted at that, since it's mostly empty space for now) when I consider that some of these volumes will be stored and synced in the cloud.

What possible reasons could BitLocker have for needing volumes to be 64 MB large, when it's not even appearing to use that space?

BitLocker FAQ on TechNet

The limit is probably for marketing/branding rather than technical reasons. BitLocker's purpose is to encrypt entire drives, rather than individual files or folders. Microsoft wants people to use Encrypting File System instead for file-level security.

p.s. Considering the smallest flash sticks you can buy these days are 8 GB, 64 MB ought to be little enough for anyone.

My guess on this one is that the overheads related to encrypting a volume using BitLocker are slightly under 64 MB, hence this lower limit, otherwise the overheads would exceed the size of the volume. These overheads include any metadata as well as some temporary space used while files are being encrypted.