Not by much ..if you've seen otterboxes on other devices the additional thickness is negligible ...visually on a n900 it may make it seem like you now own a brick.
It can't be worse than a silicone case...got one for n900.
or the pdair aluminum and neoprene-lined case...got 2 - 1 for n810 1 for n900.
and it can't be worse to a proper leather book-case..
love mine...3 leather pdair book-cases - 2 for n800 and 1 for n900
and a boxwave for my n810's with 2 card slots and slots for debit' credit, ident. .. protective plastic skin over keyboard. ...(best book design ever frankly)

Naw the otterbox can't be worse than any of those aesthetically...
and frankly I like the protection...
I know over the years I have dropped every device I have....repeatedly....Thank goodness for my cases...Never walked around without a case on any of them. All are in stellar condition....Thanks to my cases.
Don't care for the lack of protection of the silicone's open face...nor having to get the thing apart just open the device back.
Don't care for the lunky flip-up hinge on the aluminum case..awkward ...but it is effective
Love my book-cases though. classy ... and great protection and access.

If a soul's got a Nxx0 ....
I Highly suggest getting a case of some sort...
.