It depends on your room size and what your TV volume preference is. If you have a small room and usually watch TV at a lower volume, 40%-50% might be a little higher than usual. But if you are in a larger room/space and you like to watch TV at a higher volume, then it is pretty normal.

As long as the volume is consistent from all of your sources and the audio sounds good, that’s all that really matters.

To compare, I have a 20 x 13 ft room with 8 ft ceilings and I usually have the volume set to 25%-40%.

I would say we have a decent size room- definitely not small, so I guess 40-50 is probably nothing to worry about. 

But the thing that made me question it is before I got my Arc setup I was just using the TV internal speakers and never had to go above 30 or so on it. I was thinking it would have been the opposite- that the Arc would not have to be turned up nearly as much as TV speakers.

The Arc is processing the same source through multiple speakers arrays. Left/Center/Right as one array.   Then possibly an Upward firing array and maybe a Side firing array depending upon the source material. 

The Arc processes the sound for clarity and not volume which is a user adjustable feature. You do have an option in the Sonos App to enhance dialog. You can also use Trueplay (if you have an iPhone) to tune the Arc to the room environment; which may help as well.