All things being equal, small grains will have a greater collective surface area than large ones for the same weight of charge. Thus the charge burns faster which results in a faster rise to a peak pressure. The larger grains will burn for a longer time so distributing their energy over both a longer period and in a further expanding space as the load will have moved further up the barrel by the time the gas has expended. The extremes are small pistols (say .32 calibre) which may cope with a 4F charge and a large cannon (say 6" diameter) which will blow itself up with a 1F charge. Hence coarse cannon powder is sold to hobbyist cannon users and large 19th century military cannon used grains aptly termed 'pebble powder' for ones which were over an inch in diameter. Complicating it all is the heat energy which continues to expand the gasses even after they have ceased to be produced. The Swiss Aubonne black powder factory makes it's sporting powder with slightly more nitre than the classic chemical proportions used by ordinary powder makers to include more oxygen to enhance the heat expansion rather than just gas production on deflagration.
It is very hard to isolate one factor in black powder deflagration in use in such a complex material. I could also mention density, glazing (as opposed to the detrimental use of graphite for aesthetic marketing purposes) purity, wood and non wood types, creosotes and temperature of firing, pressure and length of incorporation and gas encapsulation, I will stop there as life is too short. The role of sulphur is still a field of study even today.
The ultimate grain is, of course, the solid black powder pellet which was coming into military rifle use just as smokeless powders were beginning and the shape of the pellet (in rod form for modern centre fire @8mm bore cartridges) was critical for formulating the shape and peaks of the pressure curve from ignition to expulsion and that technology became that of modern solid fuel rockets.
Note well that substituting modern solid rocket fuels for black powder in any form in a gun is not a sound idea for the backyard experimenter at all. No exceptions. Even if you are very clever and definitely not if you are just stupid and brave.
Even black powder pellets are a risk outside industrial testing. The British .303" cartridge was designed for smokeless powder but production of that was not ready when the Lee Metford rifles began production so a solid black powder pellet was used for a short while instead. The peak pressure of the black powder cartridge was noticeably greater than the replacement cordite smokeless powder cartridges and this was after extensive research and development into the black powder pellet. It is easy to make one with a sharp spike of pressure that will exceed the strength of the action.
