The size classes, plainly

Single-wide. One section, delivered in one piece. Typically long and narrow. Lowest cost, fastest to site, and the easiest to place on a difficult parcel.
Double-wide or multi-section. Two or more sections joined on site. Wider, more conventional room proportions, and closer to how a site-built house feels inside.

The jump from single to multi-section is not only square footage. It changes delivery, the foundation, sometimes the financing category, and often what a given community or county will accept.

Comparing two homes honestly

Square footage is the number everyone leads with and the least useful one on its own. What actually separates two homes at a similar price:

Wall construction and insulation. Ask about the thermal package, especially if you are somewhere with real winters or real summers.
Roof pitch and load rating. Matters for snow, and for how the home looks from the road.
What is standard versus optional. Two homes at the same list price can differ enormously once the second one has the upgrades the first included.
The warranty, and who honours it. Manufacturer and dealer obligations are not the same thing.

Walk it if you can

Floor plans do not communicate ceiling height, natural light, or how a hallway feels. If a dealer has the same model or a close relative on the lot, walking through it for ten minutes tells you more than a week of comparing PDFs.