Not quite. If you look at the roleplaying statistics for both Mon Cal cruisers and Star Destroyers you'd see that while the Mon Cal cruisers were more manoeuvreable and had more shields, the Star Destroyer beat it hands-down in the areas of hull armour and weapons.
Now, while this is perfectly OK in the movies and such, it would suck to play the Rebels in a computer game if the biggest and best of your ships could be squashed like bugs by the Empire. So therefore the Mon Cal cruisers (and the other Rebel ships) were beefed up to make battles more interesting. In any conflict between gameplay and realism you have to let gameplay win, otherwise it won't be much fun.
"We think in language; therefore, the quality of our thoughts can only be as good as the quality of our language."
-- George Carlin