Question:
Why the area is wrong in OptiSurface?  OSD file says that the acreage is 55.03 ha but in reality, it's 5.1 ha.

Answer:
Either of the two below has been selected:

You must have accidentally selected feet as the units when you imported.
Solution:  Import the survey again but select meters as the units. 

Or, it must have surveyed in feet and you have imported as meters.
Solution:  Check the survey file.