Even high-end gamers have gravitated to smaller-form-factor PCs, which typically take up less room and consume less power.
However, theres no widely adopted format for SFF motherboards smaller than MicroATX in size. VIA has its Mini-ITX form factor, which allows just one expansion slot, while Intels Micro- and Nano-BTX form factors have received little support outside of Intel-branded products.
AMD is stepping up to the plate, offering the DTX standard. DTX is strictly a mechanical specification based on the older ATX form factor. The standard will be completely open, and royalty-free. Its also not specifically tied to any AMD products.