I wrote this post quite a while back, I’d say you’re correct – the size difference is more from the electronics and screen they need to fit in there on a digital camera.
However, I do believe I was correct that in terms of compact cameras with APS-C and larger sensors, that the difference in how a sensor and film deal with light at oblique angles typically results in larger lens designs being needed for digital than were needed for film compacts.

Is it, though?
I’m probably wrong here but I’m having a hard time believing that the difference comes from the imaging medium because of systems like Canon EF or Nikon F where the same lens with the same flange distance works with film or digital bodies equally well, yet if you compare the sizes, on average the film bodies tend to be smaller than the digital ones still.
Feels like most of the size differences comes from things not directly related to imaging, like buttons/controls, screens, card slots, microphones and the like – I think most film cameras only have a plain film door in its back and nothing else?