It doesn't just sound lower in pitch, it
is lower in pitch.

3 of the 4 strings are tuned the same as the violin, but there is a string below the lowest violin pitch and the top violin string is not included. So violin is G-D-A-E, but viola is C-G-D-A.
Also the sound quality is somewhat different - much mellower and less brilliant. It can also be somewhat louder than a violin owing to size and the lower notes and is considerably heavier than a violin, even though the body is usually only a couple of inches bigger. I can hold a violin very easily with no arm support, but my viola 'pulls' a lot further down, (although I can still do without arm support, but I daren't risk it for very long!). For such a similar instrument, there are quite a lot of differences!!
Never tell a violist s/he just plays a big violin though......
(A 'small cello' if you must!!!

)