Waveguides and optical fibres
Microwave waveguides are generally made as small as possible while maintaining performance at the frequencies of interest.  This is done by selecting a propagation mode and designing the vertical and horizontal dimensions to allow that mode.  Typically the dimensions are on the order of 3-6 wavelengths of the frequency of interest.  This also offers control of the transmitted signal in other ways, e.g. polarization.
An optical waveguide designed to these standards would be difficult to handle without damage, and likely to twist in inconvenient ways due to the bend radius being different in different radial dimensions.  However, a typical fiber optic cable doesn't need to work in a specific mode for normal industrial uses like communication--just getting the photons to their destination is adequate without worrying about polarization, delay spread, etc.  The relatively disorganized total internal reflection paradigm is thus functionally adequate.