Using MacHg 1.0b3 on my MacBook (10.6.8).
I created a local repository for an HTML Help guide (~41 mbytes, 1219 files). I right-clicked to "Add these files".
The spinner ground away for a while while it was "Generating Differences...", then I saw "File Difference Size Limit Exceeded..." in the "Differences" pane.
The files seem to have been added - they all have green "+", and I was able to commit them (Cmd-Shift-S) without incident.