It is best to charge lipo's at 1C, so you would charge that 4500mah pack at 4.5 amps. Or the 5000mah pack on 5.0amps. If your charger can charge the packs at full power, then it should take roughly an hour, or hour 15 minutes.
I also find that weird that the higher ma pack is lighter... Could it possibly be because of the taps used (or are they the same?) Or the connectors maybe?
Edit: Got beat by TexasSP!