I have almost the same problem. Image for me resizes to 100x100. Since your original post, I'm pretty sure they updated their API, so now it is 100x100 when it was 90x90. The Facebooks documentation is terrible and they need to update it to reflect this resizing.
It says that you can reach 50x50, but why would you do it in the world if they resized the image to 100x100, which, in turn, significantly affected the image quality.
In short, the answer is that you are doing it right, and Facebook is resizing the image, and the documentation for og: image is incomplete / incorrect.
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