Like any other places...you can negotiate. SO how banks work, they pay back the dealer an amount depending on factors. Going to give you an example :
Between 10-15k financed: dealer gets back at 4.99% ( 500$) at 5.99% (750$) and at 6.99% ( 1000$).
Between 15-20k financed: dealer gets back at 4.99% ( 600$) at 5.99% (850$) and at 6.99% ( 1100$).
Between 20-25k financed: dealer gets back at 4.99% ( 700$) at 5.99% (950$) and at 6.99% ( 1200$).
Numbers I made up so you have an idea, not real numbers. So chances are that a bigger place ( for having more volume) are willing to negotiate easier...
All this to say, small place, big place, all get same rates and same kickbacks. It's up to you how you manage to get 4.99% instead of 6.99%.
But it is logic that at a bigger amount financed, let's say 40k financed ( at 4.99%) , they get same kickback as financing 20k at 6.99%. So that too might be a factor in the rate you are getting...
BMW finance, would only finance a certified car.
If the BMW is not certified, they can not finance it, so any other banks ( Scotia, BMO, RBC, etc) offer it at 4.99 to 6.99%