Janeen,
I use Daz Studio a lot and what I do to pump up breast size is use the injectors that have been created by WillDial. You can get them for free at Renderosity.com in their freebie section.
So all you have to do is register for a free account at Renderosity
http://www.renderosity.com (totally free, doesn't cost anything) and then look around in their freebies section. I think it is called the Free Stuff section. There are lots and lots of goodies there, but you'll want to do a search for "Ultimate Breasts". Then you'll find all of the injectors that Will Dial has created that are available for free there.
He's made injectors for Victoria 4, Aiko 3, Stephanie Petite, and others.
Its pretty easy to use the injectors/morphs. I use them a lot and if you want to see what they end up looking like you can see on my deviant art page
http://grycat20.deviantart.com I have both FMG (Female Muscle Growth) and BE (Breast Expansion) stuff on my DeviantArt page so I'm guessing you'll want to just skip the FMG stuff and look at the BE stuff there - if you do that you'll see what sorts of effects you can achieve with for example Victoria 4 if you use the WillDial Ultimate Breasts freebie injectors - which is what I have been doing.
If you are more ambitious and want to do some actual sculpting / modeling yourself and make custom breasts, you might want to go the Poser route rather than Daz Studio and I think some folks here like RickyJava and PoserLover can probably give some advice on whatever they are doing to make their models come out with such fantastically huge breasts.
I'm not very good at doing that raw modeling stuff though, so I've taken the lazy route and have been using the premade Ultimate Breasts injectors from WillDial and they have worked pretty well for me.
capper (aka grycat20 on deviant art)