Is Pikachu on a diet? I noticed that Pikachu evolved a little over time.
I wonder why he changed his form so much, so I'm a bit worried about Pikachu.
It's most likely a matter of different character designers over the different generations of the franchise. The original design was created by Game Freak's (original game's designers) character development team and finalized by artist Ken Sugimori. The games themselves had changes in Pikachu's designs, which were in turn mirrored by the anime:

 "Green and Red", 1996

 "Blue", 1996

 "Yellow", 1998

 "Gold", 1999

 "Ruby and Sapphire", 2002

 "Diamond and Pearl", 2006

 "Platinum", 2008

 "HeartGold and SoulSilver", 2009

Ken Sugimori is also one of the character designers for the first TV series from 1997. In 2002, Pokemon Advance had Sayuri Ichiishi as the character designer. And in 2006, Pocket Monsters and 2010's Black and White has Toshiya Yamada in charge of character designs.

 from the "Advance" generation