Have you actually made your own test swatch? I knit a lot looser than many pattern gauges, so I often can get away with using thinner yarns than a pattern calls for. If it's still too small try moving up one needle size and see if that works. You might get lucky.

Annie

Might I ask what summer top pattern you are using? DamascusAnnie is right about the swatch. That's the first thing you'll want to do. I personally knit tighter than the given gauge. If the pattern is a simple one and includes a schematic you can use your gauge to personalize the sweater with whatever yarn you would like. I just did this with a dress recently and I used Grace yarn.

Sometimes the only problem with using larger needles when the yarn is of smaller gauge is that the weave in the fabric tends to be looser making the garment a little more "revealing" than one would like.