Small pak have more tiles on the screen, it mainly depends on the number of tiles. If you zoom in so a tile has the same size (regardless of "native" size) then you get the same FPS.
Also simutrans has to update vehicle movements, but the smallest unit of movement is 2 pixels. Hence running at too high fps will not improve smoothness of the display. Furthermore, if there is much demand on displaying, the actualk interaction loop will be reduced, so reaction on clicking will become much slower. There is really no point in running simutrans faster than 30 fps. Many servers force either 15 fps or 25 fps on their clients.
