Why doesn't Pikachu look like he used to?

Look at Pikachu's design from 20 years ago, and he looks almost unrecognisable compared to his design in the modern Pokemon games. So what happened to this so-called "fat Pikachu"?

NOTE: I referred to Pikachu throughout this video as "he", since Ash's Pikachu is male. I am aware, however, that Pikachu is a species, not a specific character.

PATREON -   / thomasgamedocs  
TWITTER -   / thomasgdocs  

SOURCES
- https://web.archive.org/web/201807110...
- https://web.archive.org/web/201805042...
- https://www.polygon.com/2018/6/4/1742...
- https://nintendosoup.com/ken-sugimori...
-   / 1126657377636212736  

MUSIC
- "Kitchen Bossa" by Kazumi Totaka
- "Machinations and Deductions" from The Great Ace Attorney Chronicles
- "Investigation" from The Great Ace Attorney Chronicles
- "Ryunosuke Naruhodo - Overture to Resolve" from The Great Ace Attorney Chronicles 2: Resolve
- "When" by Stephan Siebert
- "Prelude to Pursuit" from The Great Ace Attorney Chronicles 2: Resolve
- "Jingle - Intermission (Resolve Version)" from The Great Ace Attorney Chronicles 2: Resolve

Other music via Epidemic Sound

CHAPTERS
0:00 - Intro
0:34 - PART 1: The original Pikachu
4:37 - PART 2: Pikachu's new design
8:49 - Outro