jon, you may be right about higher loads on the lowers but certainly not for the reason you used above. When the mast is upright gravity isn't pulling it to the side, the wind is. There isn't as much sail area up high as there is down low which I suspect increases the lateral load on the lowers.ralph, lay you mast on it's side. you pick it up from the area around the spreaders while wifey picks it up at the head of the mast. who is carrying all the weight?
and ralph, most everyone is smarter than i am, but i did do my homework.
I have a single spreader rig with forward and aft lowers. I set the tension in the uppers first then the forward lowers approximately equal to the uppers while keeping the mast straight laterally (athwartships) then I tighten the aft lowers. On my previous boat I just hand tightened the aft lowers but I find I need more than this on the "new" boat to keep the mast from pumping. The two lowers add up to more tension than the uppers.
@Ted I like the free body diagram. When you talk about compression loads at the mast base it is worth considering the tension in any halyards led to the deck. In the "pedantic" category, it may be worth noting the sail load is lower at the head and higher at the foot of the sail. A lot of the load is taken by the boom which is attached low on the mast. This would certainly increase the lateral load at the mast base. If your intent is to pictorially represent the loads it is a good illustration. If you are interested in calculating loads, it would be worth more accurately representing the sail loads.