Thanos was beefed up by Marvel Comics in order to look more like Darkseid. When the Mad Titan was originally conceived, he was based on the New God, Metron. However, creator Jim Starlin has a conversation with editor Roy Thomas, he decided to bulk him up in order to appear intimidating like the big bad Ruler of Apokolips. While Thanos wasn't based on Darkseid, he was certainly inspired by him.
Darkseid first appeared in Superman's Pal Jimmy Olsen #134 and was created by Jack Kirby in 1971. Two years later, Marvel Comics debuted Thanos in The Invincible Iron Man #55 by Jim Starlin. Both menacing villains would become the ultimate villains in their respective universes while seeking out ways to shape reality in their image. There's been rumors ever since over whether Thanos was a copy of Darkseid, but that's not true. Instead, Starlin used Darkseid as inspiration but based the character on the New Gods observer Metron. But, Darkseid's influence would ultimately help shape what Thanos looked like.
In an interview with Jon B. Cooke (via CBR), Starlin discussed creating Thanos and how he planned on introducing the character. Starlin complimented Kirby and the "terrific" New Gods series at DC, while admitting the concept for Thanos was inspired by the now-classic series. He said in early drawings. Thanos was inspired by Metron - who was much skinnier than how his character turned out. However, after talking with Marvel editor Roy Thomas, he was encouraged to beef him up, as Starlin recounts him saying, "If you're going to steal one of the New Gods, at least rip off Darkseid, the really good one!"
Thanos would appear in The Invincible Iron Man #55, where his influence from Darkseid (including his boots) was apparent. He would eventually evolve into an even bulkier character and become arguably the greatest villain in Marvel Comics history.
It's bizarre to think about what Thanos could look like without Thomas' advice to make him beefier like Darkseid. Although, Marvel Comics would later introduce Thanos' son Thane, who was a much skinnier version of his father, and perhaps, was inspired by the original Metron connection.
Thanos and Darkseid are two of the most iconic villains in comics and the fact the Mad Titan was inspired by the DC New God will always connect the pair. Thanos was never a rip-off of Darkseid as some falsely state, but Starlin clearly admired Kirby's work and was inspired by it to create him - even if he originally looked more like Metron. Thankfully, Thomas told him to beef him up, leading to the creation of the Thanos readers know today.
Source: CBR