Casio’s boxing series of game calculators was released in 1981 and had them step away from number games for the first time.
The BG-20 was different to the others as the larger screen meant Casio could extend the graphics to extend below the waist of the boxers, and show the crowd gathered for the fight.
Functionally it was the same as the other boxing calculators, no extra features for the extra size, and this time the calculator was definitely secondary to the game.
Casio used this form factor in the BB-10 and BB-101 baseball calculators, and several non-calculator games which had calculator equivalents — including the SG-11 Soccer and PG-100 Pachinko games.
Rarity varies on this calculator, they crop up from time to time and prices aren’t too outrageous. If you really liked the BG-15 in the 1980s, it’s worth trying to find a BG-20 to give you the perspective of the game with full-length pugilists.