Just came back home and thought I'll give both Tanchjim Sapce and W4 a quick measure. Both are under the same 47ohm dummy load, max volume. Note, we are only looking at max voltage here (and its associated power), and ignoring that the distortion could very well affect the actual rated power)
Space:
3.5mm: 0.69V x 95mA = 65.55mW (USB Power Draw 0.72W)
4.4mm: 1.38V x 107mA = 147.66mW (USB Power Draw 1.18W)
W4 (High Gain)
3.5mm: 0.68V x 87mA = 59.16mW (USB Power Draw 0.72W)
4.4mm: 1.31V x 149mA = 195.19mW (USB Power Draw1.07W)
As you can see, W4's 3.5mm has actually no advantage over Sapce's 3.5mm when both are maxed out. It offers a tiny less volume and power. On the 4.4mm, however, even though W4 actually has less volume, it gives more power because of more current. So here is a situation where you don't always get more volume even where there is more power.
Space:
3.5mm: 0.69V x 95mA = 65.55mW (USB Power Draw 0.72W)
4.4mm: 1.38V x 107mA = 147.66mW (USB Power Draw 1.18W)
W4 (High Gain)
3.5mm: 0.68V x 87mA = 59.16mW (USB Power Draw 0.72W)
4.4mm: 1.31V x 149mA = 195.19mW (USB Power Draw1.07W)
As you can see, W4's 3.5mm has actually no advantage over Sapce's 3.5mm when both are maxed out. It offers a tiny less volume and power. On the 4.4mm, however, even though W4 actually has less volume, it gives more power because of more current. So here is a situation where you don't always get more volume even where there is more power.