Relationship between CPU Usage and Battery Usage
Long story short, battery comsumption depends not only on CPU but also every other part of your device.  

Full version. An SoC consists of CPU, GPU, modem, north bridge, south bridge and RAM (on a discrete semiconductor chip but packed together into the same shell) altogether. Also the battery powers not only the SoC but also sensors, screen (which is usually the most power-consuming part) and wireless connection antennae.
A CPU has its own algorithm to control it's power and performance, which is called a 'governor'. A radical CPU governor tends to provide better CPU performance at the cost of high battery consumption, whereas a gentle governor will try to reduce battery consumption, resulting in lower CPU performance. The same applies to GPU and other parts of the SoC.
The screen is often the most battery-consuming part of a device, and the most decisive factors of a screen's power are size, resolution, brightness and technology (LCD or OLED). Cheaper phones and watches use LCD screens while high-end ones use OLED screen, as OLED uses significantly less power than an LCD one of the same specifications.
A watch usually has significantly less battery capacity but not-so-little screen power, which will decrease the draining difference in its battery stats while the screen is on.  Besides, the sensors used on the phone and the watch may not be exactly the same, causing various differences.
The bottom line, the battery cost is determined by all parts of a device rather than only CPU. If another part is draining too much battery, you can't find large differences even though the CPU's power is highly variable.