I've worked as customer service at one of these budget phone companies before. The batteries are decent — I've had my fair share of battery bloating for certain models, but the battery life is definitely comparable to the bigger brands.
Sure, but the budget phone also isn't pushing a quad HD screen and a beefy processor. The runtime may be similar, but that doesn't mean the capacity is.
I did a quick straw poll by looking at some budget Android phones against some flagships and it seems like the budget batteries are about 1000mAh less than the flagships. In a bigger case. So, bulky and low capacity.
But the flagships I'm aware of have non-removable batteries which is not a fair comparison as swap-ability has pay offs that make up for all the plastic and more robust circuit designs.
You're right.
I'm not sure how much variance LiPo technology has when it comes down to cost vs quality though. We're arguably paying for better battery QC, but I haven't read a news article about someone being injured from a battery exploding in a budget phone.