There seems to be a lot of confusion about sweater bras, also known as t-shirt bras. Many women assume them to be padded bras that will make them look bigger. Not necessarily so! Padded and sweater lined bras are very different.
Padded bras are designed to add dimension, usually a whole cup size, but sometimes more or less.
Sweater bras have a thin lining in the cup, either molded foam or molded spacer fabric. They are not designed to add dimension, but are designed to solve many other problems. Sweater liners add modesty, support and usually smoothness. They avoid nipple show, give extra support in the cup, mask slight side to side differences, help take pressure off shoulders and camouflage padding in one side as needed.
A firmer foam padding adds enough support & shape in the cup to allow less coverage and a narrower strap even up to a J cup. In a non-underwire bra foam lining replaces the function of wires and seams.
The lighter weight spacer fabric is softer, cooler and molds to the shape of the breast better than foam but still adds coverage, smoothing and support.
Looking bigger or smaller has more to do with the shape that the cup is molded into. Some are flatter and almost minimizing while others are more pushed up and projected.
The only way to see if foam or spacer fabric cups can solve your specific issues is to try various styles where fitters can help with the evaluation process.
Keep an open mind!