It seems like each wood type has a "large" variant - Jungle, and now Taiga (Spruce) and Oak - not Birch yet, and I've no idea what they plan with Acacia - they're jungle wood with oak leaves... so it would seem fine to leave the variant as a 2x2 of the original, at least to me... personally, I like the idea that you can build spawnable structures in the world by arrangement in the world rather than on a crafting table - like double chests, golems, and Nether portals...
This is not a duplicate of the other bug - it's not related at all to which block you are on when crouching - it can be any block type. If you are adjacent to a 1/16 block, you will fall to the 1/16 block which will deactivate crouching. It is not the same bug.