Why Decibels compare two power levels using dB = 10 log10(P2/P1). Going from 20 W to 200 W is a ratio of 10, and log10(10) = 1, so the result is 10 dB. A factor of 10 in power is always 10 dB, no matter what the starting wattage is.
Watch out The larger values like 18 dB or 28 dB would mean much bigger ratios (about 63 times and 630 times the power); people sometimes pick them by wrongly subtracting or adding the watt numbers instead of taking the ratio.
Power times 10 = +10 dB, times 2 = +3 dB, half = -3 dB, tenth = -10 dB.