Most OEM batteries meet their claims, which is why I always remind people about Seidio's exaggerations.
If you are putting in a battery that is 2x the thickness of the OEM extended battery on your phone, but you're only getting about a 40% increase in capacity (2100 mAh vs. 3000 mAh), that should be factored in when considering the size difference, that's all.
Also, it's important to temper expectations about what battery life improvements will be like. Again, since the OEM battery ratings are usually accurate, and Seidio's are usually inaccurate, you have the
expectation of nearly doubling the OEM extended battery's life, with the reality that's nowhere close.
I find it ridiculous. The OEM batteries are usually extremely close to their rated capacity - there's no excuse for these aftermarket batteries to be so far off.
This was done by some guy last year:
BatteryBoss Calls Out False Capacity Claims!