Did Pikachu lose weight during the series?

If you look at him from the first episode and compare how he looks today, he looks A LOT thinner. In the beginning he looked kinda chubby. So did Pikachu lose weight, or what?

He does look skinnier, but I think it's more due to character design drift than to actual weight loss. If you look up Pikachu's sprites and official game art and such, you can see the same thing happening there.

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.

I wouldn't say he lost weight , but rather a change in style and character design. It's mainly cause of what Hikari said, the change from cel-shading to digital coloring has to do with it.

He was definitely pudgier at the start of the series, but like mentioned above. It's not that Pikachu 'lost weight' it's his design from the artist sprites.

maybe kanto has richer berries?

While it is because the art style has changed ever so slightly, my headcanon is that Pikachu has lost weight due to traveling with Satoshi for so long.