It was the width of a horse that kept man (or at least, US men and women) in space.
Roman roads were built to accommodate two horses harnessed side by side. Naturally, the carts and chariots they pulled were sized to fit on the road. Those carts dug ruts in unpaved roads, which over time became a standard. When railroads were invented, the rails were similarly spaced because early railroad cars used off-the-shelf axle assemblies. Although the trains themselves overhung the track, there's a limit to how wide and tall they could be and still be stable. Train tunnels and bridges over the tracks had to allow enough room for the biggest trains to fit.
Flash forward to the space age, and the solid rocket boosters had to be small enough to fit through the tunnels, as even segmented they were too heavy for roads and had to be transported by rail from Utah to Florida.
So ultimately, it was a horse's ass that limited the shuttle's payload. ;-)
