what does the size() option on bootstrap do?

I apparently do not understand the documentation for the bootstrap command.

Code:

. sysuse auto, clear
(1978 automobile data)

. set seed 42

. bootstrap e(N) e(r2), size(30): regress price weight
(running regress on estimation sample)

Bootstrap replications (50)
----+--- 1 ---+--- 2 ---+--- 3 ---+--- 4 ---+--- 5 
..................................................    50

Linear regression                                           Number of obs = 74
                                                            Replications  = 50

      Command: regress price weight
        _bs_1: e(N)
        _bs_2: e(r2)

------------------------------------------------------------------------------
             |   Observed   Bootstrap                         Normal-based
             | coefficient  std. err.      z    P>|z|     [95% conf. interval]
-------------+----------------------------------------------------------------
       _bs_1 |         74          .        .       .            .           .
       _bs_2 |   .2901023   .1183702     2.45   0.014      .058101    .5221036
------------------------------------------------------------------------------

.

What I expected was "Number of obs = 30" and _bs_1 to also be 30, with no variation. Instead, bootstrap has apparently used size(_N) which is the default, and given me 74 observations.

Can someone explain to me what effect the size(30) option had?