Aside from the size limit another reason for this message is that the file is encoded with UTF-16. Atlassian SourceTree does not properly recognize unicode and UTF-16 files as text which makes them unviewable and uncomparable under SourceTree (specifically git diff).
If the file suffix of the UTF-16 files is known (*.uni for example) then all files with that suffix can be associated with UTF-16 to UTF-8 converter with the following two changes:
Create or modify the .gitattributes file in the root directory of the repository with the following line:
Then modify the .gitconfig file in the users home directory (C:\Users\yourusername\.gitconfig) with the following section:
[diff=utf16] textconv = "iconv -f utf-16 -t utf-8"
These two changes should take effect immediately without reloading the repository into SourceTree. It applies the text conversion to all *.uni files which makes them viewable and comparable like other text files. If other files need this conversion you can add additional lines to the .gitattributes file. (If the designated file(s) are NOT UTF-16 you will get unreadable results for that file.)
The conversion process is invisible and does not affect or touch the file itself.
Ultimately Sourcetree relies on the 'git diff' command for that information. Typically you will see that if the file is binary or encoded as something other than ASCII.
You can see what git thinks the diff is by opening the Terminal and running git diff against that file. See https://git-scm.com/docs/git-diff