Passthrus are 5V because USB is 5V, but it doesn't necessarily pull 5V, because the voltage is only part of the equation. Whether it's a battery or usb, what matters the most is how much power (watts) are generated. I'll show you what a 2.6 ohm carto does on batteries and usb chargers.
With a battery, the watts(P) are determined by the known battery voltage(V) and carto resistance(R). From there you can computer the amps(I). The equations are: P = V^2 / R and I = V / R.
3.6V generates 5.0W at 1.4A (bloog battery)
4.2V generates 6.8W at 1.6A (non-bloog battery fully charged)
With a passthru, it's limited by the amps. P = I^2 * R and V = I * R. Once the voltage actually hits 5V, going higher with the amps means nothing because the carto is pulling all the amps it needs.
0.5A generates 0.7W at 1.3V (this is standard usb 2.0)
1.0A generates 2.6W at 2.6V
1.5A generates 5.9W at 3.9V
2.0A generates 10W at 5.0V (max volts are reached, more current means nothing)
So let's pull all of this together. To get the same experience on a passthru as on your battery, you need 5W. With a standard usb port generating 0.7W you aren't going to be vaping. With a 1A charger, it's going to vape fine but there's no throat hit at all because it's half as cool as on the battery. At 1.5A, you are running it hotter than a bloog battery, but cooler than a fully charged non-bloog.