Stored power (Wh) would be a more useful measure of capacity than mAh, but the latter seems to have become the normally quoted measure. They are not equivalent because power depends on voltage as well as current, and thus on e.g. electrode materials. So it's not a given that say a 3,200 mAh pack actually stores more power than a 3,000 mAh one if they have different battery chemistries.
Then you have to consider, as Slug says, that a device using a less efficient SoC, more power-hungry display, less efficient radio transceiver or poorly-optimised software could very easily outweigh the difference in battery capacity. So I understand where you are coming from, but I'd not get too fixated on a single number that's only one part of the equation.
That said, Lenovo have in recent years released a couple of dual-SIM handsets with very large batteries - for example, the
higher-end K80 meets your quoted specs. I've no idea what its actual performance is like, because these things tend to not make it into Western markets.
I would however avoid phones from unknown brands quoting superficially amazing specs at low prices - you really do get what you pay for, or a little less.