No, there is absolutely no difference between paid and unpaid downloads. They link to identical ISO files.
To answer your first question, yes, elementary OS is open source. Most of the code can be found here. Even the elementary.io website itself is open source.
To answer your second question, you're making a mistake by confusing MB with MiB. It's a pretty common error. Here's the difference:
- Decimal: 1 MB = 1000 KB
- Binary: 1 MiB = 1024 KiB
The website itself displays the size in MB, while your application actually displays the size in MiB. The size is the same, the standard used to display it is not.
Here's an example. I have downloaded the
.iso file using qBittorrent. Then, I checked its size using both qBittorrent and Pantheon Files. Here's what I got:
Notice how they are displaying the different size even though they are actually displaying the size of the same file.