Question

explain why do smaller cell have better efficiency

Answer 1

Small cells have larger ratio of surface area to volume and therefore the rate of diffusion and transport of nutrients across cell surface is faster. Faster rate is important for cells to function in an efácient way as it provide nutrients which are necessary for them to function. When cells gets bigger, their ratio of surface area to  volume decreases and thus rate of diffusion and transport of nutrients become slower.
Therefore, cells become less efácient