reduce (something) by (something)
To make something smaller, weaker, narrower, etc., by some amount, degree, or margin. The government has reduced our budget by nearly 25%. Even reducing the temperature on the thermostat by a few degrees could save you hundreds on your energy bill each month.
reduce something by something
to diminish something by a certain amount. I have to reduce your allowance by two dollars per week until you pay me back for the broken window. I will reduce the bill by a few dollars.