  We present a compact iodine-stabilized laser system at 633 nm, based on a
distributed-feedback laser diode. Within a footprint of $27\times 15$ cm$^2$
the system provides 5 mW of frequency stabilized light from a single-mode
fiber. Its performance was evaluated in comparison to Cs clocks representing
primary frequency standards, realizing the SI unit Hz via an optical frequency
comb. With the best suited absorption line the laser reaches a fractional
frequency instability below $10^{-10}$ for averaging times above 10 s. The
performance was investigated at several iodine lines and a model was developed
to describe the observed stability on the different lines.
