The breathing issue is also connected (although a little more tenuosly)
to this length-volume relationship. In the absence of a centralized
respiratory system in insects,
This is also true for myriapods*, but not for spiders. Spiders have "book
lungs" which transfer oxygen from the air to the blood. Mwahah! Perhaps
there's a limiting factor in there -- haemocyanin(e?) (a more or less
turquoise pigment that contains copper) binds oxygen less efficiently than
haemoglobin (the vertebrate stuff, red, with iron... even though the iron
isn't the reason for the red color!).
As for the lack of an endoskeleton in arthropods: insects do have the
so-called tentorium in the head.
* Millipedes, centipedes, a few smaller groups, and probably
arthropleurids -- the things that reached a length of ~ 2 m in the
Carboniferous and are believed to have munched rotten wood or something,
though I don't know why.