Answer: Believe it or not, some cultivars of hibiscus are more vigorous than others and are different sizes at maturity.
The difference in growth rates could be attributed to the difference in cultivars, but might also be because one has become established in its new home and the other one is still trying to adjust. Was the soil similar when you planted them? Continue to treat them equally, prune a bit to encourage more growth on the smaller one, and exercise some patience.
Hibiscus is subject to snow scale. Snow scale gets it's name because it looks like powdered snow on the branches. Scale can be found at the base, at branch junctions and on the leaves. A general insecticide such as Orthene, Insecticidal soap or a light spraying of vegetable cooking oil can control this scale. Make sure you use a light application of the oils and do not apply during the heat of the day. Hand grenade scale looks like a tiny brown helmet attached to a branch. This can easily be controlled by dislodging it. Large infestations are best controlled by removing the branch. Keeping the snow scale under control avoids the hand grenade scale.
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