"Or that I'd end up working just as hard, but it wouldn't feel as hard?"
It seems like there's no easy answer to your original question Pooks, in part because there are so many variables beyond the weight of your bike. A heavier bike does takes more energy to move than a lighter bike all other things being equal. But your questions seem to imply that the weight of the bike is the only thing that affects how hard your workout is, and it's not. On any given bike, including a road bike, you can make your workout hard if you want to make it hard by riding it in a bigger gear, into the wind, up hills, at a faster speed, faster cadence, etc. Having worn my HRM on my road bike, I know I get a good workout regardless of how "smooth as silk" it may be.
The lighter weight of a road bike and, more importantly IMO, the position it puts you in, makes a road bike more efficient than your cruiser, which is one of the reasons it might inspire/allow you to do longer and faster rides and, ultimately, burn more calories, build more muscle, etc. I know you said your goal was to get a lighter bike that fits, but beyond that, what are your cycling/fitness goals? A lighter, better fitting bike might inspire you to ride more often and for longer stretches, but at the end of the day, you have to be motivated to ride and to work hard at it when you do.
But here's my experience for what it's worth. My first bike was a hybrid, and I road it about 300 miles before getting my road bike. Less than a year later, the road bike has about 3,000 miles on her. And the hybrid? Still about 300.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher