Splenda (as most other powdered sucralose products) contains 95% (by volume) bulking agents dextrose and maltodextrin.

This is why you use about the same mass/volume of powder to sweeten your coffee as you do with sugar despite sucralose being 350-1000 times sweeter than sugar.

If you used just straight sucralose, it is conceivable that you could use 1000x less product than sugar to achieve the same result.