If a radio ham is running bare foot, in other words 100Watts, then an increase to a 1KW amp, would be a 10dBW improvement.
The “S meter” on the radio is in dBV, so a one “S point” increase is a 6dBV improvement in signal.
A 10dBW increase of signal, from bare foot to a 1KW amp, is 20dBV improvement, which is just a little more than a three “S” point increase on the radio signal strength meter, or from S9 to +20dB over.
A transmitter output of 25Watts, is ¼ of running bare foot of 100Watts, which is a 6dBW less than bare foot power. This is a 12dBV difference, which amounts to a two “S” point difference.