Yes, you can. No, they haven't
First of all, I'll like to say that YES to can have more capacity in a same sized battery. Lithium-Ion (or Li-Ion Polymer) batteries can be made using different electrode and different electrolyte. Depending on the choices you make the volume metric energy density can range from about 200 to 650 watt-hour/liter. The stock Sony EP500 is about 385 watt-hour/liter, so is pretty decent but not exactly as good as is technologically possible.
However, I must also say that I have not seen one 1600, 1800 or 2340mAh battery that is not complete, utter, frauds. I have sampled three and all are similar to or worse than the stock battery. First of all, 1800 and 2340 is simply unattainable using Li-Ion chemistries, period. About 1750 is about as high as is theoretically possible if you don't care about costs, durability, safety or anything else. Even 1500~1600 is highly dubious. Let me explain...
First of all the EP500 is a Li-Polymer (aka Lithium-Ion with solid polymer electrolyte). Li-Polymer batteries are easier to make in a prismatic pancake and/or a flexible pouchcell format. They also have a solid electrolyte which means that they don't have liquids that can boil and cause an explosive pressure build up -- that's why Li-Ion batteries have a pressure relieve valve that blows to vent the battery so it will fail but not blow up. However, Li-Polymer is not as energy dense or capable of as high a current draw as traditional liquid electrolyte Li-Ion designs.
Another thing about Li-Ion type batteries is the discharge cycle. Li-Ion batteries degrades the least if you keep it between about 25 and 75% charge. A lot of times, a battery is rated for 1000mAh because that is how much it'll carry if you call 20% the minimum and 80% the maximum for instance. By limiting the maximum charge and discharge you prolong the useful life of the batteries. Electric systems in cars like the Prius and Volt does that too... The volt never discharge past 20~25% and nevver fully charges to the past 75~80% of the batteries actual capacity. Why? Because that's how that get that $10000 battery pack to last past the 8-year/100K mile warranty period! Similar measures are use on cell phone batteries to get the battery to last at least 1~2 years in normal use.
So, if you want a higher capacity EP500 sized battery you'll switch to a Li-Ion (vs Li-Polymer) chemistry, use the strongest electrolytes, charge it closer to the theoretical max and allow deeper discharging. Do that and you can probably get to 1400~1500mAh in the same size package. The price you pay is that the battery will be more expensive and has a short service life -- maybe 6 months instead of 1.5 years of typical use before it falls to 50% its "like new" capacity. Also, you better know what you are doing with temperature and pressure management during charging and discharging or the battery may blow up! Lastly, there is additional problem that your chemistry may produce slightly higher or lower nominal voltages -- Li-Ion chemistries range from 3.5 to 4.2 V in mean voltage (EP500 is about 3.9V) -- so it may or may not be compatible with the phone's charging system.
Anyway... to sum it up... yes, you can get to a 1400~1500 mAh EP500 sized battery. No, you cannot get to 1800~2340 mAh. Such performance carries a cost and longevity price and may or may not be voltage compatible to the phone. And, no, none of the "High Capacity" batteries I have seen so far have done anything sell lies and bullshit.