Smaller sensors always make a lens frame as if it were longer, because using a smaller sensor is, in effect,cropping. See also this image demonstration from dpreview's original 5D review.
645 film has a crop factor, relative to 135/35mm format (so-called "full frame"), of 0.614x according to this sensor format table on Wikipedia. The crop factor is the ratio of the formats' diagonal measurements.
You can figure this out with the Pythagorean theorem (a2+b2=c2), if you know the sensor dimensions. 645 is 56mmx42mm, and has a diagonal of 70mm; 135 is 36mmx24mm, and has a diagonal of 43mm. 43÷70 = 0.614. The different aspect ratios of the two formats, however, isn't going to make this exact, despite the numerical precision. :D
So, while a 50mm lens remains a 50mm lens no matter the size of sensor that's behind it, on a Sony full frame (135 format) sensor, if you want an equivalent field of view to that a 50mm rendered on a Bronica ETR, you need a 50x0.614 => 30mm lens.
And the field of view the 50mm will yield on full frame will be equivalent to a 50/0.614 = 50x1.63 => 81mm lens on the Bronica.
So, if you're used to focal length/FoV translation on the ETR, then on the Sony,
- the 50mm will frame like an 81mm would on an ETR camera
- the 75mm will frame like a 122mm would on an ETR camera
- the 150mm will frame like a 245mm would on an ETR camera
- the 250mm will frame like a 407mm would on an ETR camera
Again, this won't be exact, since the two formats have different aspect ratios. But it's close enough for jazz. All other factors, DoF, perspective, etc. would look identical if both cameras were shooting the same subject with the same lens from the same distance. The reason larger formats appear to yield thinner DoF and different perspectives is mainly because with a larger format, you'll more typically either shoot closer to the subject or use a longer lens to get identical framing with the smaller format.
Footnote: Film 645 format, btw, by medium format shooters is also "full frame" while digital 645 is crop. :D 645D, as the sensor format is known, is only 44mmx33mm, and has a diagonal of 55mm, so its crop factor, relative to 645 film is 70mm/55mm = 1.27x. The 645D format is used by Pentax's 645 dSLRs, as well as Fuji's GFX cameras.