In doing the math, 28 watts to 1500 watts is 17.3 dB gain, while 5 watts to 150 watts is 14.8 dB gain, and 5 watts to 200 watts is 16.0 dB gain.  The point is the amp appears to be operating normally with your numbers.    Ideally, the gain of a linear amplifier is constant up to the point of compression nearing maximum output.   To better understand this, more accurate numbers would be needed.   And it is not at all unusual for an amp to have different gain values on different bands. 

Simple calculations support your observations:

dB =10log(P1/P2)

73

Bob, K4TAX