Given the same lens placed on a 4x5. 5x7 and 8x10 you will get the exact same picture, the only difference is that you "see" more if the image circle in the larger formats.
Since the smaller format "cuts" a lot of the image circle if you want the larger format to show the exact same crop then you would need a linger focal length, this is the reason the focal length changes as the camera gets bigger.
When I worked in a camera shop we had a rule of thumb to come up with the equivalent focal lengths. We would multiply by .3 or multiples to get the approximate focal length, for example:
210 x 0.3= 63 if I wanted to go from 4x5 to 35mm
210 x 0.6= 126 for medium format etc.
It is not very accurate but it gets you in the ball park.