sync: Use correct type in size calculation

The previous and current types are the same size, so this wouldn't
have caused a bug in practice, but it is confusing, and would have
been a problem in the unlikely event we changed the size of one of the
types.

Test: sync-unit-tests on bullhead
Change-Id: Ic43b81f3b4ff214af86b6b6d4d02c648f95d0c4b