In my opinion (opinion because I don't have anything other than experience to back up my claim), both small and large batches take the same amount of time. That said, there are many variables that most people don't consider when assuming a small batch takes less time. One is that many people will still pitch an entire packet of yeast (5 grams) into each batch. This means that there are more yeast cells per milliliter of must. This fact gives the small batch an unfair advantage.
Another factor is in how much nutrients are being added. Are your nutrient additions proportionately stepped down? Or did you guess the difference, or worse yet, add the same amounts? This will have a huge impact on the speed and quality of your fermentation. I once had a complete fermentation in 3 days by using the same amount of nutrients in a 1/2 gallon batch as I normally use in a 5 gallon batch. Though the fermentation was quick, the flavor was undesirable.
As I've stepped up from hobbyist to professional (though sometimes awkwardly), I've made a couple observations.
1. Even large batches of mead can be made as quickly as smaller ones.
2. Watch your fermentation temperatures when stepping up batch size. The ambient temperature is not the temperature of your fermenting mead. Temperatures can greatly affect the performance of your yeast, and especially affect the flavor profile of your mead.