Fish finder is right. It's all about volume, small body in big bag, they won't have the physical volume to warm the air in the bag.
I say buy once from someone like Kathmandu, Paddy Pallin etc.
I prefer down bags, more $$ but have had our current bags for about 15yrs.
You need to consider, will they get used much or possibly get wet and dirty? Down is not so good when wet. Synthetic will keep you warm if wet.
I would not buy sleeping bags from somewhere like a caravan based retailer, Rays Outdoors etc.
I use mummy shaped bags and once you get used to the tapered shape, they are great. Also warmer because of less volume.