First thing its the smaller the cheaper, getting a smaller engine and kit cuts the price a little.
And of course the biggest reason, "the bigger the are the harder they fall" (if thats true with rc planes). Seriuosly im telling u right now i know im gonna eventually crash its happens, one of the biggest worries of getting into this hobby is building the kit spending the $$$ and then destorying my plane of my first run.
On a smaller plane if it crashes the impact wont be as big and a $20 repair can be made and ill be back and flying in no time. however having that huge plane (70" is huge to what i expected) is gonna crash and destroy itself on my first run.
By the way whats the max speed for the sig LT-40 (u can use imperial system for this one,MPH, lol)?