but even a regular midtower sized dell or hp pc has a proprietary motherboard, proprietary power supply, other pieces... and there are industry standards for mini-itx motherboards (170x170mm), microatx motherboards, mini and microatx cases, and power supplies that are designed for use in common mini-itx size cases. and also standards for low-profile expansion slot height microatx size cases.

the lenovo m78 I have which I was using as an example is no smaller than an equivalent spec pc built from standardized parts.