f you are getting the "Unable to calculate the differences. The files have too many differences, there aren't enough resources to complete the operation" message, it means that you have reached the maximum differences limit.
The limit is 32k differences per operation. The file size shouldn´t be a problem.
It needs more or less 1,5Gb memory. We decided to limit the max differences because after this limit the memory increases a lot (we have the algorithm optimized until these values).
In order to reduce the differences, you can also try changing the comparison method in the external merge tool. This way, the differences number should be reduced.
In the Plastic "Preferences" panel, you can also configure a different merge/diff tool.
