Controlled indoor lighting vs outdoor available daylight

Hey!
I'm a bit unsure if I chose the correct subforum for this post, also I feel this is a very noob question so feel free to laugh :D

I'm shooting a lot of interview stuff indoors with a standard 3point controlled lighting setup.
Usually a fast prime lens, with settings similar to these: f/4, 1/50s, ISO 160 (lowest), if I try to go more open with the f-stop the picture easily is overexposed unless I counter with ND etc. What bothers me though is that I get quite a bit of grain and low quality image even though I make sure there's enough light hitting the sensor, so exposure is balanced. This is indoors. Once I go outside in bright daylight and use very similar settings I get a much more cleaner result.

Why is that?

I'm guessing that because the distribution of light is much more even outside it gives a better image?

So what should I do? Get much brighter lamps and counter overexposure with much smaller apertures?
Or will indoor footage never look as clean and even as outdoor stuff shot in daylight?

Thanks!

ETA: Oh, fwiw I'm shooting 1080p, 24fps on a hacked GH2, lenses vary but above is observable despite which lens is used.
 
Can you show us one of these images? 160asa at f4 is quite a lot of light, so maybe you're unknowingly underexposing the image?
 
Can you show us one of these images? 160asa at f4 is quite a lot of light, so maybe you're unknowingly underexposing the image?

Sure, I could upload some 100% cropped stills. What do you mean by unknowingly underexposing? I always make sure the exposure indicator is right on the money...
 
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