Is there a word for "unnecessarily large"?

I would like to say:

The utility produces unnecessarily large amounts of data, therefore most of it is discarded immediately.

The word should also, if possible, express the position that not only is the amount too large for me, but that it is too large for any reasonable use.

Excessive seems perfect in your case. Otherwise oversize but that would apply more to an item than a mass of data.

Superfluous suggest the idea of excessive and unnecessary: (from TFD)

• exceeding what is sufficient or required.

• the utility produces a superfluous amount of data.

The utility produces a surplus of data.

surplus (noun) pl surpluses. "an amount (such as an amount of money) that is more than the amount that is needed" Merriam-Webster. - "an amount, quantity, etc., greater than needed" The Free Dictionary

• There is a surplus of workers and not enough jobs.

• China's trade surplus has caused...

• Lake Cayor on the right side of the lower Senegal and Lake Panieful (Guier) on the left constitute reserve basins, receiving the surplus waters of the river during flood and restoring them in the dry season.