What is the difference of Compact, Mid-size and Full frame size DSLR

What is  the difference of Compact, Mid-size and Full frame size DSLR

There are basically three different sized DSLRs. I'll use Nikon as an example. 

All SLR cameras must have a mirror and a reflex viewfinder. ***

Compact SLR: A "compact SLR" is usually an entry level product such as a Nikon D3xxx or D5xxx. however some mid range products are quite small too, such as nikon D90, Olympus E-30.

Compact SLR Nikon D5200

Mid-size SLR: Mid-size SLR are usually semi professional products. This SLR usually with metal alloy construction and light weather sealing.  Nikon D7100, Nikon D300, Nikon D700, Nikon D800 are good example. as is Olympus E-5

Mid-size SLR Nikon D7100

Full size SLR: Full size SLR are professional products, with strong construction, and full weather sealing. These are the Nikon D1, Nikon D1x, Nikon D1H, Nikon D2, Nikon D2H, Nikon D2Hs, Nikon D2X, Nikon D2Xs, Nikon D3/D3S/D3X, and Nikon D4 etc

Full size SLR Nikon D4