Your right about having to cut some of the running line off, or going to a fairly short shot of backing to get the larger size RIO's on to a lot of reels. The RIO spey lines are a minimum of 120 feet long (vs. 90 for a 'standard' line) out to 150 foot in total length.
Not sure why over 120 foot as you'd have to have the timing of an Olympian, the rod lenth of a tree, and the arm strengh of a weight lifter to get that much 10-12 wt grains airborne. There are folks (one's nick name is "long cast Steve" who can do this cast after beautiful cast. For most of us the cast really have 70 to 90 foot airborne (dry lines) plus 12 to 17 feet of leader. So the 120 feet of main line I understand, but 150 foot?
On with the rant: Another thing I think we get fiddled with is the amount of backing we're advised to have on a reel. In salt applications, wide slow moving waters, you're in a boat and can chase the bloody thing. So lots of backing could come into play.
In normal river fishing condtions, if Mr Fish has out 120 foot of fly line, plus leader, plus only 30 yards of backing that fellow is over 200 foot away, new gets another 30 yards .... and still moving .... and well on his way to spooling you. Math says he's at least a full football field away and "still going and going, and going..." Probably why I only use 20# backing and never over 12-15 # main line tippet(Max. Ultra Clear, like who doesn't?) for leader material. At least this way I'm not putting a $70 to $120 fly line 'on the line' when I hook a salmon.