Gabby and Penny are both big girls, with Gabby more muscular. Tall Penny looked in good proportion beside the even taller Ian, whereas Gabby does not have that advantage. It is only when the two girls stood together that you noticed Penny is almost a head taller.
As for Gabby's dance steps being efficient but funless, well amateur Gabby can only dance the steps choreographed by James. Ian appears to have put a little more humour into his choreography, and in the dance-off side by side Penny's dance looked softer with more sparkle, Gabby's like a nonstop inventory of moves.
Perhaps to compensate for the absence of humour and softness, Gabby was at pains to maintain a wide smile throughout her windmill action of limbs, which appeared to have backfired.