Small Disk Or Partition Better Than A Large One Full Of Files?

I am trying to get several peoples opinion. I believe that a small disk that is bootable similar to a ssd is better that a large one full of data. There is less disk to deframent and it should be faster. The larger data disk will contain most of the files many of which will not be used a large part of the time. One can keep larger files such as pictures or videos in that data disk. I probably prefer a small ssd(less than 100GB) but am not sure that is the best way to go right now. Later on when the price and quality of ssd goes down I could substitute that drive for the small hd partition or drive.