First, I would check the manufacturer specifications in case there is a recommended minimum volume for accurate readings.
Second, I'm not sure but isn't the beampath in plate readers vertical? If so, with such difference in volume you are changing considerably the path length on the sample, which according to the Beer-Lambert law will affect to the absorbance:
A is the absorbance of the sample (no units)
e is the molar absortivity with units of L mol-1 cm-1
b is the path length of the sample - that is, the path length of the cuvette in which the sample is contained. We will express this measurement in centimetres.
c is the concentration of the compound in solution, expressed in mol L-1
So, unless the instrument is capable of measuring the path length (height of sample) the volume will really matter and if it can, your volume could be below the instrument operation limits