How are you transferring the data? Did you set up an EC2 instance and run the DataSync agent there? If so, you might need to allocate more memory.
If you're doing it without an EC2 instance, I think you're right — the bucket might be too large. You should consider filtering the data and transferring it step by step, especially if it's a one-time operation.
If this also does not work you may need to contact support - in my experience DataSync behaves weird sometimes and I had multiple cases where the support had to fix something for me, before it worked.