  The discoveries of huge amounts of dust and unusual extinction curves in
high-redshift quasars (z > 4) cast challenging issues on the origin and
properties of dust in the early universe. In this Letter, we investigate the
evolutions of dust content and extinction curve in a high-z quasar, based on
the dust evolution model taking account of grain size distribution. First, we
show that the Milky-Way extinction curve is reproduced by introducing a
moderate fraction (~0.2) of dense molecular-cloud phases in the interstellar
medium for a graphite-silicate dust model. Then we show that the peculier
extinction curves in high-z quasars can be explained by taking a much higher
molecular-cloud fraction (>0.5), which leads to more efficient grain growth and
coagulation, and by assuming amorphous carbon instead of graphite. The large
dust content in high-z quasar hosts is also found to be a natural consequence
of the enhanced dust growth. These results indicate that grain growth and
coagulation in molecular clouds are key processes that can increase the dust
mass and change the size distribution of dust in galaxies, and that, along with
a different dust composition, can contribute to shape the extinction curve.
