The Tamron 28-75 f/2.8 is a highly regarded lens.. at $350 it's definately worth it.. I had a Tamron 17-50 when i first started out and i loved that thing.. It had it's quirks though, noisy AF motor and poor AF in dark conditions but for $400, i was willing to put up with it.
Some lenses are bigger than others because of their design.. lenses are like cars.. the purpose may be the same but are not all the same inside. There are luxury lenses and there are cheap bang for the buck lenses just like cars.