Because NiCad batteries for electric motors are heavier than fuel tanks for gas planes, in order to achieve good performance from electrics, some care must be taken to minimize the weight of the structure of the airplane. Fortunately, most gas plane kits are hideously overbuilt. Thus one can reduce the structural weight of the plane so that the electric plane will not be significantly heavier than the gas plane.
Often the structure can be lightened 50% since electrics do not exhibit significant vibration and do not need to be fuel-proofed. Coupled with lightweight radio equipment and cobalt motors, electrics can yield identical performance to their gas-powered counterparts.
When building a model, question the role of every piece of wood. Could you remove the piece entirely or construct it out of lighter materials? The adage "build strong where needed, and build light everywhere" should be foremost on your mind. Remember, a lighter plane flies better -- and a lighter structure does not mean a weaker plane!