ditto-- what's wrong?
are you just talking about resolution? That will, in a fashion, make things bigger or smaller-- but generally the highest resolution will be the best looking. A lower resolution might look zoomed in and 'bigger,' if that's what you prefer.
If you're unable to change resolution... I think under video drivers, there is a default setting to allow individual programs to set resolutions. So that when I watch a non-widescreen movie, it can auto adjust. But I vaguely recall that there was a way to not allow programs to set their own resolutions, forcing it into 1080p or whatever you set it to. Such that when, for example, I played Riven (old old game), it forced it to 1080p and was quite tiny. That shouldn't be the default setting; but I think it can be set-- which might prevent sims3 from changing the resolution to what you want.
There's something else called overscan/underscan. That would not show up for us on a screenshot! What would happen, is that the projected image would not reach the borders of the screen itself, leaving a wide black border around a smaller image (if it were underscanned).
Is that what's happening?