thumbnail byte-larger than original

This issue has been migrated from [#2667](https://github.com/matrix-org/synapse/issues/2667).

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### Description

The thumbnail for an image I uploaded ended up bigger than the original in bytes at the same number of pixels
### Steps to reproduce

- Uploaded the image that's at mxc://matrix.org/nlkUlJCMIBWDWZLKjxZYknrn (123kB)
- Download the thumbnail at mxc://matrix.org/VPkLkCQZUMycjuCrARzGbkKa (204kB)

Describe how what happens differs from what you expected.

It probably would have been better not to produce the thumbnail in the case where the result was larger.
It's a good question why it's larger; both are the same res (513x405) - I think teh thumbnail added an alpha channel for no reason; but is that the reason it's so much larger?

### Version information

- **Homeserver**: matrix.org just now