johnny1000:
In the case of a 75/25 percent split, classic style, the division point is where a large block is inserted into the chain. From that point, the 75% will mine three times faster than the 25%. The 75% of miners will not mine on both chains, every miner will have to choose. The blocks after the large block can be other large blocks, 1MB blocks, or blocks with only coinbase transactions - it doesn't matter. The distinction is only if the miner can tolerate that there is this large block buried at some point in the chain.
Therefore, the 30% chance of finding the block first at the crossroad, does not flow forward to the next block or the next block after that. The 75% threshold is so big that there is practically no risk of splitting the chain. Also don't forget there is a grace period, so the fraction may be closer to 99 percent. My guess is 1) there will be not even an orphan 2) if there is, the chain will stop there, 3) if not, there will be at most a chain of two blocks before the smallblock chain is given up. After all there is a cost to mining, and the risk of losing the reward and the fees increases cruelly for each block mined in the main chain.
Unless someone wants two chains, that is. But this can not go on for long, for economic reasons. There is too small difference between the two chains to make both useful.