If I could place the RX100 sensor in A500 for bird shooting, .........

But the exposure properties have not changed.  Set ISO 100 on an A77 and it is as sensitive light as the A99's sensor when set to ISO 100.   So what you do not have to do is open your lens up more on the A77 to get a correctly exposed shot when you set the camera to ISO 100 compared to when using an A99.

If there was a change in exposure properties then you would have to use different settings for aperture and/or shutter on an A77 and A99 when they were both set to ISO 100.  That is clearly not the case.

Now I am sure someone will say because the A77's senor is nosier then the A99's then there is indeed a loss of sensitivity with the smaller sensor   Even that isn't correct because whatever the physics of it as far as noise goes that does not define how sensitive the sensor is.  This was also true in the days of film.  Films of the same speed had different amounts of film grain.

I remember an using ISO 64 slide film made by Perutz.  It was as grainy as hell but gave some fantastic colours (and it was cheap!).  Compared to Kodachrome 64 which was virtually grain free it was far grainier (noisier in today's terms).  It was however equally sensitive.

Bottom line is the "film" in a Nikon V1 is as sensitive as the "film" in A77 is as sensitive as the "film" in an A99.  If it were not you would need to set wider apertures or slower shutter speeds when using the V1 and A77 to obtain a correct exposure and you do not.

Also consider external exposure meters (yes they still make them and very sophisticated they are too).  These would be impossible to use and obsolete if sensitivity varied with sensor size and smaller sensors were indeed less sensitive.

It has not changed period.  There is no equivalence issue at all.

Resolution is not the same as sensitivity and it is the sensitivity of the sensor that dictates how much light is required to obtain a correct exposure.

At ISO 100 you will set F5.6 and 1/1000 for EV15 on any camera with any sized sensor.  Basically whatever size the sensor is then the "sunny 16" rule will always apply.

It doesn't matter so long people do not start thinking they can only use their small sensor cameras in blinding sunlight because they have been misled that they require much more light to operate in.