Cheap products are almost always made cheaply. The quality of the mixes used to make batteries will vary greatly which can result in lower capacities for the cheaper made versions which can also increase the weight of the product as well. Even a quality manufacturer creates and then grades their batches with the higher quality batches getting a higher price. That of course doesn't mean there are not lower cost batteries that aren't made with good quality. Which is why such a generic question can't tell you much since the answer also must be generic.
The people here often have favorite brands that they only get from reliable vendors so they know they are really getting the product they are being told they are buying. There are a lot of fake branded products. Amazon is awash with every kind of 3rd party vendor from reliable to pretty sketchy. Like buying anything, doing proper research into both the products and vendors often pays off in the end.