That's a complicated question. None of this is from my personal experience, but much reading on the subject as I considered switching to Provera. My understanding is that those two doses are equivalent for the purpose. As in, you can't really compare them directly because they just aren't the same, but 100 mg of Utrogestan or 2.5mg of Provera is prescribed for daily continual use. I think Provera is a "stronger" progestin than the micronized progesterone in Utrogestan, so a very tiny 2.5mg does the same work to your endometrium as 100 mg of the other. Also to consider, I believe the Utrogestan is not as consistent in how it is metabolized in the body, so the amount you actually "get" may vary from another person or day to day for yourself.
It may be that the Utrogestan wasn't doing a great job at keeping your lining thin, so the Provera is catching up. If you feel well, I would note it and watch it for now and expect it to settle soon.