The ones you grew are not actually the real big variety. Just look up "huge tomato varieties" and you will get many responses. A variety I once grew was Big Zac and they were huge. There are many others.

Browse around there are plenty of info for newbie. Usually everything starts with really good soil... here is article I like but there is more and Soil forum is informative.
Size of variety plays big role too.
Miracle grow generally produces lush foliage and few tomatoes. The company is not known to care about health of the planet or quality of their products- my opinion.

Here is a link that might be useful: quality soil

What about taste?

Another consideration is not only the size of the fruit, but the number of cells and the meatiness (as opposed to a variety which has large cells and a large proportion of gel: a good beefsteak variety compared to the large-celled Rutgers, say).

You will never get a really big tomato unless you grow a variety that produces big tomatoes. There are lots of these available to you. Take a look at Belgium Giant and Delicious, both heirloom and Big Zac which is a hybrid. Look at the Totally Tomatoes catalog which has a section dedicated to giant tomatoes. The seeds are out there but then again, what about taste?

Thank you for the responses! I have been getting the gist that miracle grow is not preferred, and also the sorry excuses for products you can accidentally buy in the gardening aisle. (I just bought 'Neem oil' in a hurry only to realize that it was 1% neem oil and 90% pesticides that kill bees and are toxic!) But that's a whole different thread...

My 2 early girls are in half-barrels, with miracle grow potting mix and my other 2 are in a raised bed in 1 part peet, 1 part bagged compost and 1 part bagged garden soil from lowes.. there is a grape tomato plant inbetween in the raised bed that produces like crazy and is huge, all flowers, but the tomatoes are gravel sized! I am going to read on soil now, thank you

Thank you for the responses! I have been getting the gist that miracle grow is not preferred, and also the sorry excuses for products you can accidentally buy in the gardening aisle. (I just bought 'Neem oil' in a hurry only to realize that it was 1% neem oil and 90% pesticides that kill bees and are toxic!) But that's a whole different thread...

My 2 early girls are in half-barrels, with miracle grow potting mix and my other 2 are in a raised bed in 1 part peet, 1 part bagged compost and 1 part bagged garden soil from lowes.. there is a grape tomato plant inbetween in the raised bed that produces like crazy and is huge, all flowers, but the tomatoes are gravel sized! I am going to read on soil now, thank you

I dont need them to be huge, just the size they should be. I'd like to see them reach their full potential. I am excited for next year's garden to really research taste and size before plopping the first green thing I see into the ground ;)

My Early Girls and Grape Tomatoes

My recent most favorite tomato fertilizer is Texas Tomato food. All I can say is wow about it, great stuff. One has to order it on line though unless in the area, but customer service is superb and delivery is in couple of days.

Here is a link that might be useful: tomato food

1- Variety is the first factor.
2- Care and growing conditions:
---- This includes a lot of things : Temps, Sun, Soil, Ferts,
Climate, Watering,
3- Reducing burden on the plant. Obviously, a plant growing wild, no trimming, no pruning, with zillions of flowers, tiny fruits.. is not going to produce many large fruits.