If bitcoin popularity grows, more transactions with lower per-tx fees will likely result in more total fees per block, compared to a world in which the block size is fixed and individual fees are higher. The induced demand will be a significant factor. This is fairly straightforward economics. This is a good thing by the way =)
Regarding orphan rates: I don't understand all of the gritty details, but I have seen some convincing explanations describing how, if block space isn't scarce, a miner's decision to include an additional tx is largely based on the likelihood that adding the tx will result in a block they find being "orphaned". So it is a self-regulating feedback loop of sorts. And as another poster said, it should affect all miner's equally, assuming they have a sufficiently sophisticated setup (low-latency, well-connected, etc).
For those don't know about orphan rates: An "orphaned" block is one that is wasted (and the reward thus lost). This happens when another miner finds a competing candidate block at a similar time and manages to propagate it more quickly through the network such that it is chosen by the majority as the next block.