Facebook - Specifying Image Sizes?

Question

Looking at Facebook I have noticed that images vary in size from 100x100 to upwards of 300x300 (and in some cases larger). I realize some of this can be done using Open Graph yet when I specified the image size for a photo from my site it takes the photo and reduces the size to a smaller size. This is okay to a point as the image size is originally 620x541 yet it reduces it to a very small size (roughly 100x100) which makes the text on the image very hard to read and in some cases the image deteriorates to looking like a very crappy photo.

Why is it and how is it that some photos are 300x300 (roughly) yet FB takes my image down to a much smaller image? I have used OGP to specify the image I want pulled yet is there a way to tell FB not to reduce the image below a certain size? Here is my OGP I used for my image:

<meta property="og:title" content="My Title" />
<meta property="og:type" content="article" />
<meta property="og:url" content="URL Path" />
<meta property="og:image" content="Image File Path" />
<meta property="og:image:type" content="image/jpeg" />
<meta property="og:image:width" content="620" />
<meta property="og:image:height" content="541" />

Answer 1

Here is why, answer is credit from Nick over at the Webmasters Stack Exchange

The reason some of the photos on your wall appear larger than the linked photo is that they have been reposted from one of your friend's photos that was initially uploaded to Facebook. i.e. They are not shared links to external websites.

Facebook shows your friend's photos with a large thumbnail, but external sites with a small thumbnail. There's no change you can make to your site to get it to use larger thumbnails. It's just how Facebook works at present.

If you wanted the images to appear bigger in other people's streams, you could upload them as images to your Facebook page (or to an official Facebook page for your site), and get people to like the photos you've uploaded to Facebook directly instead of liking the link to the page containing the photo on your site.