I've been wanting to share this. The doctor's holocam is rather obviously a R/C car radio. In fact, I used to have that exact radio. It's from the Airtronics Avenger line. It's a low-cost 2 channel ground radio for RC vehicles. Nice and rounded, for the 24th century! http://www.airtronics.net/airtronics_Avenger.asp It was a cool prop, but I giggled cuz it was obviously my RC radio.
Im currently re-watching Voyager and everytime I see his holocam I can't help but think, damn that's big! I mean that far into the future you figure It would be small, at least the size of a credit card like some of todays cameras.
Since the tech for realistic holograms was new enough in "Encounter at Farpoint" to surprise Riker (2364), I don't mind that a device capable of recording a photorealistic 3D image is not yet credit card-sized when Voyager departs (2371).
Keep in mind too that the Doc's holocam had MAJOR capabilities. It didn't just grab a 3D holo-image, but recorded (if memory serves) all kinds of spectrum information when he used it for scenic shots, and micro-cellular details when photographing patients. Think back to Latent Image. In order to record that much information about his patients, that little holo-cam had to do some pretty powerful photographing! I actually liked seeing that not all tech was credit card sized. Sometimes, you just can't make everything using nano-probes and magic fairy dust!
Quark had a much smaller (and less RC controller) holocam: http://ds9.trekcore.com/gallery/albums/3x08/meridian_291.jpg
OMG! Is that from the episode where Quark tried desperately create a holoprogram of Kira? It was HILARIOUS!