Have you set up your equipment profile in beersmith? If not, that's the first thing you should do. There is a video tutorial on Beersmith's website.
Then you need to have a handle on your extraction efficiency (I assume you're doing all-grain). Once you have a pretty good idea what that is, then you can adjust the grain amount until you reach the OG the recipe calls for. If you consistently get 90% efficiency, your beer is going to come out way different than someone who gets 65% efficiency and the OG and grain bill is based on that 65%. Another factor that influences how much grain you should use is your equipment's boil-off rate. This is why it's so important to set up an accurate equipment profile.
Also, hop AA%'s vary by year, by producer, and by farm. You might get cascade with 5.5% AA, and you might get it with 7.1%. Obviously, the higher alpha acids the hops contain, the less of it you need to use in the beer to hit a target IBU.
In my opinion I think recipes should specify grain bill by percentage, not by weight. It is up to the brewer to determine how much malt is needed to hit a certain OG. Same thing with hops - you should keep the target IBU in mind, not the specific amounts of hops. Adjust the hop amounts based on target IBU and the alpha acid percentage of the hops you will be using.