Indeed. The same rocket will give a light probe a much higher velocity than a large probe. The launch vehicle for JUICE is the Ariane 5 ECA which can launch 16 tonnes to low earth orbit[1], while the Titan IIIE that launched the Voyagers had a capability of 15.4 tonnes to a (lower) low earth orbit[2]. So the current launch vehicle is a bit more powerful, but not 8 times as much. There are some more powerful (and more expensive) rockets active at the time, but nothing 8 times as powerful as the Titan IIIE.

So NASA has the choice of 1) launching a lighter/smaller probe with less scientific capabilities and endurance, 2) spend $$$ and time developing a much more powerful launch vehicle, or 3) have a few years of patience waiting for JUICE to get to Jupiter on a slower trajectory.

Jupiter has already been visited by multiple probes so sending a small and not very capable probe would not gain us much scientifically. (But this option was taken with the New Horizons probe to Pluto some years ago.) (2) Wouldn't actually be faster and much much much more expensive (see how long the SLS rocket is taking to get done. But mr. Musk is taking this route too with SpaceX's Starship, so this will be an option in the future if all goes well). So that leaves option 3, which NASA has taken. They could probably have opted for an existing more powerful and expensive launcher to shave some years off the transit, but they preferred to wait a bit longer and not spend that money.

[1]: http://www.astronautix.com/a/ariane5eca.html [2]: http://www.astronautix.com/t/titaniiie.html

> So NASA has the choice of ...

> So that leaves option 3, which NASA has taken.

As far as I'm aware, JUICE is an ESA probe. NASA is providing one of the instruments, but I believe that's it.