Fit Difference between Brands
Once you get over a D cup, there tends to be a bit of variation in how American brands size their bras. Additionally, UK bra manufacturers, who make a most of the large-cup bras featured on this site, use another sizing range. And even if you know what your size *should* be in a particular bra, sizing varies a bit from manufacturer to manufacturer, and even between different styles made by the same manufacturer.
To start off, here's a sizing chart that I made to try to help sort things out. USA has 2 (or possibly more) sizing rubrics. Most UK bras have the same sizing system, but there is often variation in terms of some brands having tighter or looser cups and bands than others. When I talk about sizes, I am always using the British sizing scheme, unless I am talking about an American brand.
Here are some observations I've made. I'll try to be as specific as possible and let you know what I'm comparing things to when I say a particular bra seems to run big or small. I hope that this post can become a stockpile of useful information for everyone as people comment with their own observations. I should also mention that I usually wear Panache bras, so that when I say something runs large or runs small, I am usually comparing it to Panache, particularly their 2-section cup balconnette styles with a vertical seam, unless I say otherwise.
1. I've found Fantasie bras run a bit larger in the cup than all cup styles in brands like Panache and Freya. When I buy Fantasie, I tend to size down in the cup, but I still buy the same band size.
2. I've found that Panache balconettes with a 3-section cup (like the Harmony, Tango, Confetti, Inferno Plunge & Sienna) run smaller in the cup than Panache bras with a 2-section cup with a vertical or diagonal seam *or* a molded cup (like the Porcelain). So when buying 3-section cup Panache bras, I go up to a 34G, but when buying 2-section cup Panache bras, or the molded-cup Porcelain, I go down to a 34FF.
Examples of 2-section cup Panache bras are the Destiny, Casablanca, Emerald, & Tropical (google any of these names for pictures). Some Panache bras, like the Aragon, have a 3-section cup with 2 vertical seams, so they fit more like a 2-section cup than a 3-section, and you can get away with a smaller cup size. The same rules tend to apply to Panache Swimwear.
In terms of looks, Panache 2-section cups tend to give a rounder and higher shape, and make you look slimmer/narrower from the front, but your boobs stick out more in profile. So they're good for form-fitting clothes, but can give a "tent" effect with loose-fitting clothes. The 3-section cups hold you in a bit better and make you look less bodacious from the side, but are slightly pointy and look slightly wider from the front. They're both good, in different situations. I would say the former are good for more "social" situations, and the latter for more "professional" situations, but of course this is just a matter of personal preference and I'd encourage you to wear whatever you feel most comfortable with.
3. I've found Freya bras to be a mixed bag size-wise, but usually their sizing is about in line with Panache on similar styles. Sometimes their bands are a bit tighter, and Freya "mesh" bras like the Arabella have bands that seem to retain their elasticity better and longer than many Panache bands.
4. Some Goddess bras, like the smooth strapless, run on the USA (1) sizing scheme, where DDD is roughly equivalent to a British F cup. Some other Goddess bras, like the lace strapless, run on a the USA (2) sizing scheme where the cup sizes are roughly equivalent to British sizing, except E is called DDD and F is DDDD. Be sure to watch out for this! Usually, you can tell which bras are on sizing system USA (1) because they don't go up beyond DDD.
5. Chantelle bras tend to have looser/stretchier bands than other brands, and the cups seem to be low-ish and far apart. They are not a good choice for ladies with boobs that sit high up on their torsos, but the shape and the longer straps may make them ideal for ladies that have longer torsos.
6. Le Mystere bras have cups that are rather far apart, and put your boobs way over to your armpits. Normally you could fix this by tailoring the center gore in a bit, but their molded cup bras have molded foam over the center gore, making it very thick and stiff and thus pretty much impossible to sew through. I've found Le Mystere tends to make me look like a brick house... and not in the good way.
7. Ultimo plunge bras run a wee bit larger in the cup than Panache and Freya plunge bras. You might be able to go down a cup size in Ultimo if you are on the small side of a Panache or Freya plunge's cup. There is more room in an Ultimo G-cup plunge bra than in a Panache G-cup plunge like the Inferno, or the Freya "Retro" plunge. Ultimo makes a great molded cup plunge bra, if you're looking for one, though the cups are a little far apart, so you may want to take in the center gore.
There was recently a good post on 32D that talked about sizing and styling issues in several brands, from a lady who works at Nordstrom.
I would also point out that the figleaves.com comments system is often helpful in finding out if a bra is sized large or small-- as long as buyers have taken the time to comment. This is how I first found out about Fantasie's cups running large-- the commenters all pointed this out, and they were indeed right. But you have to read through all the comments to get an idea, and even then, it's not an exact science.