Let's be real. Studios is a full day park - there are 12 rides (counting Express) and 9 shows (counting Blues Brothers & Potter offerings as 1), plus a parade, a nighttime show, the play areas, exhibits, and now general Diagon Alley ambiance. For a first timer who rides 80% of what's available and deals with moderate lines, this will absolutely fill up a full day.
As for Islands, I'm of the opinion it's a half day park. True, it technically has more rides than the Studios (17 if you include Pteranadon, Dragons as 2, and Express), but several of those rides are "midway" short experiences (Storm, Doom, Fearfall, Caroseussel, One Fish Two Fish, Hippogriff, Pteranadon), moreso than the Studios (Nuthouse and Twirl & Hurl). Islands also really only has 3 shows (counting Potter offerings as 1). In general, the attractions at the Studios are longer - Disaster alone takes 25 minutes to complete once the experience begins. All it takes is some nasty/cold weather or an aversion to getting soaked to cripple Islands' overall offerings - and with no parade or nighttime show, there's nothing really to incentivize tourists to stay in this park longer. It's a half day, at best, really, assuming moderate lines.
Kong will help but it won't fix the problem. Islands really desperately needs a nighttime show and if possible some sort of daytime lagoon entertainment as well to keep guests in the park longer. More new experiences, like small Jurassic Park expansions or a new ride in Seuss (or even better, a show in the long dormant Toon Lagoon theater) will help flesh out a day. But as Skip has pointed out, Universal hasn't made moves in the way of nighttime (or daytime) entertainment over here just yet - I'll bet they figure Hogwartz and to a lesser extent Marvel/the coasters are carrying Islands for now (and they may be right). But I think as the resort continues to evolve, they're going to really need to address these problems.
