Based on my pouring in a 5 gallon paint pail of rice coal and looking at the volume of ash in the pan, I'd say about 1/3 to 1/4 the volume of coal ends up as ash. But, without weighing it, I'd say I'm getting about 10-15% ash by weight.
As mentioned above, crushing it will take up far less space. As an experiment, I used my workboot (with foot in it) to step down the ash after pulling it from my Alaska Channing III. Very gently, of course, to avoid a making a cloud in my basement. With one pressdown action, I think I compressed it by 30-40%. Of course, my black workboot was now all grey and I still got something of a cloud.
Although I would like to have less volume of ash, it's really moot for me. I leave it in contractors' bags (3mil) at the curb for the trash man each week. However...we are limited to a 50 pounds of trash per week/one 33 gallon trash can, although unlimited recycling paper & containers. Optionally, I can use $2.00/bag "extra trash" bags purchased from the town to dump more trash. Not in my lifetime! One time, I put a couple of 35-40# bags of ash into my trash barrel and he left one behind with a sticker on it reminding me of the 50# limit. Unfortunately, he has to pick them up and toss them in the hopper rather than having an automated side loader truck, so I can't sneak in a fast one. So, in the coldest part of the winter, I have to start keeping about a bag a week of ash for later curbside disposal. This year, it was in August before I got rid of the last of my ash bags from last season.
Too bad I don't have a friend in the DPW that could throw the ash in with the sand/salt used on the roads...