Hey Ian!
The localStorage used for saving is by default limited to 5MB.
You can increase the quota by opening the Javascript console and do:
webkitStorageInfo.requestQuota(webkitStorageInfo.PERSISTENT, 20000000, function() { alert("ok"); }, function() { alert("failed"); })
You will be asked for permission and it would increase the quota to ~20MBs.
Notice that this is a temporary issue that will be obsolete when we release the native application
-- which will be in some days, maybe end of this week :)
Cheers,
Oliver