A cat doesn't ovulate only once but multiple times throughout an entire 5-10 day heat period, the period of time a cat is capable of ovulating which is marked by several distinctive behavioral changes. This allows a cat to carry kittens that were conceived at different times throughout the heat cycle and even by different fathers. Simply being conceived a week after another kitten will create a different gestational age. Kittens mature so quickly that even a 1 week gap can create a vast difference in size and maturity.
Kittens will fight over the best nipples because simply put, not all nipples are created equal. Some nipples will produce more milk than others. The kittens born slightly larger will be able to fight for those better nipples and thus the size difference can become even more exasperated.
Turning the Tables
Kittens start to wean from their mothers around 5-6 weeks when they're introduced to external food sources. It's at this time when kittens don't have to soley rely on their mother's milk that we notice the growth spurt does a little dip during the learning curve but then starts growing exponentially. Any growth differences between litter mates can now start to equal out. These so called "runts" start to make a come-back.
When we look at human preemie babies the size difference and maturity rate are drastically different from their peers because of the gestational age difference. Yet, most of the time those preemie babies grow up to be comparable in size and maturity to their peers years later although they had a slow start in life. This can also be said for the "runts" in many litters. Often times they may start off in life a little slower than their litter mates but then make up for it later on and ultimately show there is no difference after all. What's also true for preemie human babies and the runts of litters is that sometimes, being born a tad too early does in fact have a lasting impact and create a slightly smaller stature later on.