For example, the Panasonic 42.5mm f/1.7 lens is visually equivalent to a hypothetical 85mm f/3.4 lens on a full frame (35mm) sensor. When I say equivalent, I mean these combinations would have the same practical field of view, the same depth of field, and—this is the part which trips people up—the same overall quantity of photons falling on the sensor area.
Historically, larger sensors have been more efficient at converting said photons into data, but the difference has narrowed greatly to the point where it doesn't matter so much, particularly at the relatively modest resolution of 4K video (approximately 8.3—8.9 megapixels) where the limits of lens diffraction don't tend to come into play.
Of course on full frame systems you can easily find inexpensive 85mm f/1.8 and very high quality f/1.4 lenses. But if you don't use the extra depth of field and you shoot FF at f/3.4 or greater, there was no fundamental benefit. (Of course you should never discount the glass advantage, as the mid-range FF glass tends to be optically superior to high-end MFT glass... but does that benefit apply to a new lens mount? Will these cameras do a good job focusing EF and/or F mount glass?)