Fluid resolution will change what the sim looks like. It's one of the tricky aspects with working with Maya fluids you start out at low res to get an idea of what it will look like then you up the resolution and you'll find your fluid gains detail but changes shape.
If your explosion becomes to small at a higher resolution try pumping a bit more density in through the fluid emitters. In terms of the tutorial it does not matter so much if your primary blast does not match the proxy object exactly it's just there for reference. You can always generate another proxy object to tweak the ground blast. (which is what i may have done I honestly can't remember)
If it's a little out don't worry about it, if it's about 10% of the size then something has gone wrong.
Fluids are tricky very small changes can make a large difference just keep tweaking it until you get what your looking for then go back and do it all again, with something this complex you'll need to go through it a couple of times to figure out whats interacting with what.
If there is anything else i can do just let me know.
From a readers' Q and A column in TV GUIDE: "If we get involved in a nuclear war, would the electromagnetic pulses from exploding bombs damage my videotapes?"