The weight lifting also triggers stronger hormonal responses due to the additional mechanical loading. Mechanoreceptors in your body will stimulate a chain reaction by way of the hypothalamus (HPG axis) that ultimately causes a ramp in testosterone and other hormones. Your body effectively has a built-in steroid dispenser that you can control.

The scale is really dramatic in my experience. The more the lifting sucks, the more your body will compensate. This trend can be non-linear for a good period of time before you begin to plateau. The tricky bit is not pushing too far and injuring yourself early on.

One interesting hybrid is running or walking with a weighted vest on. This requires some extra precautions - the vest should be very, very snug on your body. You don't want it slinging around and imposing weird lateral loads.