  While the Solar System contains no planets between the sizes of Uranus and
Saturn, our current exoplanet census includes several dozen such planets with
well-measured masses and radii. These sub-Saturns exhibit a diversity of bulk
densities, ranging from ~$0.1-3\ \rm{g\ cm}^{-3}$. When modeled simply as
hydrogen/helium envelopes atop rocky cores, this diversity in densities
translates to a diversity in planetary envelope fractions,
$f_\rm{env}=M_\rm{env}/M_p$ ranging from ~$10\%$ to ~$50\%$. Planets with
$f_\rm{env}\sim50\%$ pose a challenge to traditional models of giant planet
formation by core-nucleated accretion, which predict the onset of runaway gas
accretion when $M_\rm{env}\sim M_\rm{core}$. Here we show that many of these
apparent $f_\rm{env}\sim50\%$ planets are less envelope rich than they seem,
after accounting for tidal heating. We present a new framework for modeling
sub-Saturn interiors that incorporates envelope inflation due to tides, which
are driven by the observed non-zero eccentricities, as well as potential
obliquities. Consequently, when we apply our models to known sub-Saturns, we
infer lower $f_\rm{env}$ than tides-free estimates. We present a case study of
K2-19 b, a moderately eccentric sub-Saturn. Neglecting tides, K2-19 b appears
to have $f_\rm{env}\sim50\%$, poised precariously near the runaway threshold;
by including tides, we find $f_\rm{env}\sim10\%$, resolving the tension.
Through a systematic analysis of $4-8\ R_{\oplus}$ planets, we find that most
(but not all) of the similarly envelope-rich planets have more modest envelopes
of $f_\rm{env}\sim10\%-20\%$. Thus, many sub-Saturns may be understood as
sub-Neptunes that have undergone significant radius inflation, rather than a
separate class of objects. Tidal radius inflation likely plays an important
role in other size classes of planets including ultra-low-density Jupiter-size
planets like WASP-107 b.
