I have saplings near each other but they're 5-6 blocks apart. It was funny because i planted a bunch of saplings and waited a while for them to grow and when I came and checked on them there was only one tree. Half way through harvesting my lonely tree this monster just sprouted right before my eyes :-D I still have the same amount of saplings in the same spots and this hasn't happened again.
Heh I've got a tree story. I believe Aus was there when I had a tree grow, in a similar manner, and merge into one of the street trees. I had to call a moderator to come and chop it all out. If you're looking to grow mutant trees all the time, I suggest you plant them close together. I use a 16x16 grid. Doesn't take up a lot of space and I get especially large trees growing a lot of the time. A fair warning, however, you will be dealing with a lot of leaves to cut through.
You can force a large tree to grow by surrounding the sapling in a 2 block high wall (3x3) and using bonemeal on it.