It depends on the P&S. Most of the simple P&S have a fixed focal length lens, with no focusing adjustment possible and a fixed aperture that is usually fairly slow (eg f5.6~f8).
More complex P&S cameras have zooms lenses and slow AF systems. Typical aperture values are f4~f7)
35mm SLR camera lenses on the other hand have a considerably more complicated operation and it's normal for most lenses to have a far faster effective lens speed (eg f2.8, f1.4 etc) variable aperture control of the aperture diaphram, aperture control mechanisms, focusing mechanisms (manual focus) as well as more complicated optical designs.
Zoom lenses in SLR cameras tend to be even more complex and often have 12-20 lens elements in 5-11 groups.
Autofocus motors are frequently built in to the lens (eg. Nikon AF-S, Canon USM etc). This allows for much faster autofocusing.
The above is just a few of the reasons why P&S lenses are smaller. For more detailed information I'd suggest you search the internet.