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This shouldn't affect the battery too much because once the charger is unplugged, the phone will start using the power from the battery, but leaving it in will keep battery charged whilst using power from the socket.
In saying this, I wouldn't leave it charged on full battery for long periods of time because the battery may heat up.
Hope this helps.
I would recommend you to use the phone as usual in the day and when you are about to sleep, leave the phone plugged in till you wake up.
I cannot however comment on it making the battery big, but when you are using the device in the day and you have a good percentage of battery, I wouldn't leave it on charge because of possible heat issues.