Can you give an example? "Very large" is not very exact.
Edit: figured it out for you...
By default, Excel displays cells in "General" format. Using this format, you can put a formula into a cell and see a number displayed. For instance, =sqrt(3^41).
Under the default "General" format, this displays as 6039287738
Under the "Number" format, this displays as 6039287737.57
Under the "Accounting" format, this displays as 6,039,287,737.57
Under the "Number" format with 10 decimal places, this displays at 6039287737.5706100000
One assumes that those last five digits were simply zero filled. That is 15 or maybe 16 decimal digits of precision. Which is about right for a 64 bit IEEE float.