Sorry for necroposting, but in case any one else comes here looking for this, I have the answer. Biomes O Plenty source code is available to the public, and after checking it carefully, I have confirmed beyond a doubt that most saplings do not grow anything other than a 1x1 trunk. Sacred Oak and Redwood Saplings are exceptions, but Willow and Fir saplings specifically only grow as 1x1.
Worse, it does not appear to be possible (or at least easy) to create an addon mod that could alter this behavior. Vanilla saplings fire an event on the TERRAIN_GEN bus when they are about to grow - for these saplings, it is possible to handle that event, cancel the normal tree growth, and inject your own code for creating a tree. However, though the BOP sapling class (BlockBOPSapling) inherits from the vanilla sapling class, the overridden method which performs tree growth does not fire the event.
Working with a clone of the BOP GitHub repository, I modified the BlockBOPSapling class to support growing large Fir trees specifically. Doing so was surprisingly easy. The sapling growth method is a simple switch statement that checks the metadata of the sapling being grown and uses the same generator classes for creating the trees as are used during world generation. The deficiency is that only the metadata value is checked for a sapling and not its arrangement in the world. I added a few methods the class that check to see if neighboring blocks are also fir saplings, and if they are, the class uses the "big" fir tree generator instead of the small one with a 5-block "+" shaped trunk.
Unfortunately Biomes O Plenty is licensed as Creative Commons ND (no derivatives), meaning modified or derivative source code cannot be distributed, so I will not post the modifications here without Glitchfiend's permission. I am, however, considering opening a feature request with them.
TL;DR - You can't make many/most of the BOP saplings grow into 2x2 or bigger tree trunks.
