Value was either too large or too small for a Decimal

It means that the value returned cannot be converted to decimal as it is too large.

Decimal values can be between positive 79,228,162,514,264,337,593,543,950,335 to negative 79,228,162,514,264,337,593,543,950,335 - see MSDN.

Double can handle much larger numbers - negative 1.79769313486232e308 to positive 1.79769313486232e308. These will not all be convertible to Decimal.

Well, it's fairly self-explanatory.

decimal.MaxValue is 79,228,162,514,264,337,593,543,950,335 - your number is bigger than this.

Although decimal has a finer precision than double, double has a bigger range - it can handle very, very large and very, very small numbers.

Now, if you could tell us what you're really trying to do, we could try to help find a solution... it's rarely a good idea to mix double and decimal, to be honest.