I had a maiden this year who went late with no bag who was also huge. She never has produced a lot of milk for her colt and we've ended up supplementing him a lot. But she only got "full" the day before she had him, 11 days overdue. Keep watching for filling like you're doing, and if you can get pH test strips to watch for the decrease in level, that will give you a really good indicator of when she'll foal out. Also when the fluid goes from clear to milky white.
Best wishes!!!