I think we mean the same thing but are saying it differently. The outer legs from top to bottom seem to be large, small, large with the difference in size being just enough to let the tubes slide inside each other to adjust length and permit disassembly for transport.
I have assumed that the middle portion of each outer leg of Bob's derrick was of the smaller tube size since there is a single piece of smaller size tube without end fittings that could fit that way. Since the set of legs is not complete and they have seen some modification since manufacture (one or two flame cut ends) it is possible that the smaller diameter plain piece of tube is just being misleading and that the center part of the outer legs should be a third, even larger size that isn't in Bob's collection of parts, which would help to increase the theoretical buckling load of the outer legs which are mainly loaded in compression.
The center leg still puzzles me since I can't visualize how a top to bottom sequence of large, ????, small would fit together.
In theory, since the legs are all pin jointed at the ends, they should see pure tension or compression loads but since we don't have frictionless pullies or any of the other neat gadgets from the "pure physics toolkit" there will be some bending loads.
The diagram in the Body Parts List suggests that the center part of all 3 legs should be larger diameter than the tops and bottoms and that the tops and bottoms should all be the same diameter. This is a non-dimensioned drawing, too small for me to be sure that I am reading it correctly and with respect to the center leg doesn't match the sizes measured for top and bottom sections.
If you think I'm a bit puzzled, you are right.
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