The semi’s may seem small to some, but in 1900, the average home in the US was 1000 square feet, and I can’t imagine it was much different in Canada. Even post-WW2, houses hovered around the same size, only increasing to 1200 square feet in 1960. So at the time these were considered average sized homes. Below is a plan of a general elongated semi-detached house (or at least the right half of one).
These houses are generally well constructed, even though historically they are considered to be “economically” built. Flooring was often 1/4″ strip oak (which after 80+ years of wear does not often refinish well). Trim was a combination of oak, and likely some form of knot-free older-growth pine, or maybe Douglas fir. Doors were oak veneer over a solid Douglas fir/pine core. The fantastic aspects of these homes are the small items, like stained glass windows either side of the fireplace (and across the front living room window, sadly many of these have disappeared over the years). Not every home has these, but it somewhat works against the principle of these homes being built in an inexpensive manner.
Some people prefer detached houses, but the benefit of semi-detached houses is the common wall – especially in winter. Why? Because the common wall is in a climate-controlled environment, so does not leak heat in the winter. This contributes to lower heating costs.