No, what you get is a more wide open pickup, ie: like getting more juice......theres less resistance to the current (probably not the correct terms or the best way to explain it,as I don't know all the techno mumbo jumbo, so maybe someone else will expand on the details) but the bottom line is the bigger the pot the more your gonna hear the true pickup as it is full blast, with a 1 MEG pot being the maxed out version of your pickup and the 250 being the least (what single coils use) keep in mind just because your opening your pickup up like a governor, don't think it will translate to better sound, as most pickups are set to have the best tone with thier 500K pot (humbuckers) and 250K (single coils)or some people like it lower, say 350K or 400K for thier humbuckers, etc...though its a personal thing, so some guy's may want the Ice pick in your ear tone.............that a 1 MEG pot on a Dimarzio Super Distortion, may yield......hahahahha
You can also add resistors and change capacitor values as well to enhance and change tone, but thats a whole lot more detail, and you can find that stuff on the guitarnuts web site and others should you want to experimant with all of that.
Further note, you want to buy audio taper pots, as they taper off the volume slowly and give you that taper as it says, linear pots will not be like a traditional volume / tone pot in that respect, so buy audio taper pots, this is what most guitar parts suppliers sell, however should you be at an electronics shop someday, you now know to skip the linear taper bin and go straight to the audio taper.