Dimensional stability is crucial to possible applications of single-walled nanotubes, as their properties are linked to size and topology. We observe nanotubes responding to uniform atom loss, through surface reconstruction and drastic dimensional changes. Experiments using electron irradiation evidence nanotube diameters shrinking from similar to 1.4 to 0.4 nm. Molecular dynamics simulations show that surface reconstruction and size reduction occur through dangling bond saturation, forming nonhexagonal rings and 5-7 defects in the lattice. Nonuniform atom removal results in inhomogeneous tube deformations and local necking, and formation of linear atomic carbon chains in the nanotube body.