Two 3500 lb axles (2-3/8" dia. axle tube) is probably the most popular rating by far. Larger rated axles will be more expensive and some axles have larger brakes and require 6-lug wheels which adds to the cost. The TT manufacturers try to build TTs around 3500 lb axle ratings and can play with the CCC numbers to make it to work. Axles can be loaded to or near their ratings in some cases (after deducting a tongue wt. figure) which can increase the risk of a bent axle. (Tires can also be loaded to near their max rating.) The TT manufacturers conveniently don't list axle ratings and details in their specs.
Our 1st TT was 20' (with dual axles), had a 1500 lb CCC rating and weighed close to it's GVWR. Our current 29' TT (an "ultralite") has a CCC rating of 1563 lbs and also weighs close to it's GVWR and just one full holding tank will put it over. Many TT owners quote a figure of 1500 lbs as typical weight of all cargo fully loaded for camping. Will depend somewhat on size of family and heavier things like tools that some take with them.
The 2018 version of our KZ TT (same length & features) now has a CCC of only 1210 lbs, down from 1563 lbs. That's getting to be a pretty low CC for a 29' TT. KZ says about their weights: This weight reflects the rolling average unit weight for each model. Your trailer weight will vary according to optional equipment.
TTs designated "ultralite" usually means they're built with the thinnest, lightest and least substantial materials & components to get the most bang for the buck relative to length and to make them towable by less capable TVs.
You can't trust CCC numbers in my experience. Maybe they're more accurate in the past couple of years? Our TT weighed 190 lbs more than the factory UVW, as delivered. Not sure if it was the options we ordered and/or things they may not include in the UVW like spare tire, microwave, propane, etc. The wt. of ALL cargo for camping for two came to only 555 lbs. The wt. of all mods (shocks, Dexter equalizers, water filtration, 2nd TV, etc.) was 119 lbs. The total wt. on top of the UVW came to 864 lbs. All wts are from a commercial scale. The as-delivered wt. was well off from the "yellow" sticker number (a white sticker in our case inside a cabinet and the max. cargo wt. on the sticker at front/left outside is also way off.
Frames do not have weight ratings of any kind and there is no correlation of the rating of axles used on them. Some frames have the much more flexible design of 3 pieces of sheet steel welded together and along with a low CCC does not make for a durable TT.