It's the power rating of the charger. Big screen phones use a lot of juice and consequently have big batteries. To get these big batteries to charge in a reasonable time, the chargers are higher rated - i.e. run at a higher power.
For example, the charger for my Nexus 4 (also made by LG) is rated at 1.2 amps (I think), most 3rd party chargers, car chargers and USB connections are rated at 1 amp or less.
If the phone isn't being used, it will still charge though much more slowly. To speed this up a bit, you can turn the phone off.
If the phone is being used - particularly if the screen is on all the time like when using the GPS - then lower rated chargers may not be able to keep up and the battery level can continue to drop, though obviously, much more slowly than it would without any charger.