Some varieties do get huge while others remain compact. I like the sprawly, climby, rambling, mounding types. I'm struggling to remember the name of what I tend to grow, and I swear it's Empress of India, although everything I read says this is a small one. Alaska can get big too. Nasturtiums reglarly, year after year, take over some part of my garden, usually volunteering. They do well with plenty of water in the early weeks, keep them moist as much as you can, and the plant will get big with those great huge leaves... then when they reach that point I ignore them for the rest of the summer and they flower their heads off.

I just now did a search for "nasturtium" on this forum - looking to see if I could find the thread from this last Spring/Summer that had the most wonderful picture of nasturtiums I've ever seen (a gardener in England) - thinking that you could email that gardener to ask about the secret of his/her success. But I have run out of time - I only really had time to read one thread, but I think it might be of real help to you. It was titled "Nasturtium Growing Tips? What's the Secret?". It had a lot of useful information. Run a search for "nasturtium" and it will come up as the third or fourth thread listed. Good luck!
P.S. I had them this last year, and I'll have them again this coming Spring. They weren't fabulous (it gets too hot here for fabulous), but they were delightful all the same, especially in the Spring and the late Summer, when the temps cooled off a bit.

I meant to include a picture of mine from last year but couldn't find it, so here's a link to the thread where I posted it last August. (forgive my cruddy camera)

Here is a link that might be useful: my nasturtium pic

I grow lots of different kinds also and they can grow very differently. Some are big and bushy, some trail all over the place and others stay relatively small. My experience is that they grow bigger and better in the ground and for me really hit their stride only later in the summer and then explode in the fall.

They love the cool, foggy weather here, and I've got the tall, trailing single type naturalized all around. I also have Alaska, but the old-fashioned trailing kind is definitely more vigorous and does have a big wow factor.

I had a seedling come up this summer that I think must be a result of cross pollination between Alaska and the regular. It has bright chartreuse colored leaves, like golden feverfew or Heuchera 'Key Lime Pie'. It hasn't gotten very big or bloomed yet, but I am hoping it lives and proves something special. If it gets bigger, I'm going to root a piece and give it to Annie at Annies Annuals so she can pass it along.