The reason behind Pikachu "losing weight" is due to the changes in the art of characters over the generations.
If you look at all of Pikachu's game sprites, you'll notice it was slightly fatter-looking during Generations 1 & 2. The official art also shows these changes.
These changes in art design also carried over to the anime, like how during late Johto saga, the coloring system changed from cel-shading to digital coloring. Also, several attacks' graphics were updated as the series went on.