You have it backwards...the larger the filter the less flow resistance and the better filtering you have. This is because the water is going through the sand slower and you have more fiter surface area. It is always better to oversize the filter no matter what kind it is. As for the loss of flow, what are the pressure readings? That will give a clue as to whether the filter is plugging (pressure increase) or you're somehow sucking air after a while (pressure decrease).
Al