I noticed the same thing when I first started to nurse lying down at night with ds2 (I was never able to nurse lying down with ds1). My breasts seemed much more full and ds2 would only need to nurse from one side before falling back asleep, when before he would need both sides. Sine I started lying down to nurse ds2 also has started to want to nurse almost all night long. He needs the boob in his mouth to sleep now.
I don't know if any of this is helpful for you, but just wanted to let you know you aren't the only one! I also worried about ds2 not getting enough at night, or that his latch wasn't good when we're lying down, but it all seems to have worked out and adjusted itself on its own. My supply is still great and I don't feel uncomfortably full at night anymore.