Perhaps, as you suggested, the USB charger you have is the wrong size.
I never use my Nokia charger having gotten a USB charger. I only use the USB charger since I travel often--and a USB Charger + Car-to-USB Adapter + My Notebook allows me to charge my phone almost anytime with the minimum of carrying stuff.
In the hotel room, I plug my charger into the notebook, and the notebook into regular power.
In the airplaine, it's the same thing but I plug the notebook into the airplane power (if it's available).
In the car, I use the Car-to-USB Adapter.
I've never ever needed the normal power charger for my phone since I've gotten the USB charger. And my USB charger never gives me an electric shock.