This is an interesting topic always. Serious/obsessive cyclists are always trying to get their watts/kg from 3 up towards 6 (tour de france standard).
And they starve themselves and do no strength work for their upper body in the deluded hope this helps.
I have trained cyclists in the past (am a physiotherapist and physiologist) and recommend they regularly test themselves (at least once a fortnight) on the same hill climb (5-15 minute climb)....if they are losing too much weight, their times will deteriorate....and the log will be enough proof to their own minds, that there is an optimum balance between body wt and power.
Getting hung up on how much you weigh is a slippery slope. Not everyone has the genetic makeup and the body frame to operate efficiently while weighing only 125lbs. I'm a firm believer that one must maintain a weight that is fairly natural to what their body frame and genetics allow. I'm 5'9" and about 143 lbs and ran a debut marathon of 2 hrs and 36 minutes, which is significantly superior in relation to any of my 10k or 5k race times where I weighed under 140lbs. One must remember that one of the world's best distance runners ever was Steve Prefontaine, and he was 5'9" 152lbs. Hardly your typical elite endurance size.