Kevin Systrom, the co-founder of Instagram, testified on Tuesday in a landmark federal antitrust trial that he left Meta in 2018 because his company was denied resources.

The government has argued that Meta purchased Instagram in 2012 as part of a “buy-or-bury strategy” to illegally cement its social media monopoly by killing off its rivals. Last week, current and former Meta executives testified that the social media giant, formerly known as Facebook, used its deep pockets to invest in Instagram after its purchase.

In testimony at the U.S. District Court of the District of Columbia, Mr. Systrom painted a different picture, saying he left Meta because Mark Zuckerberg, the chief executive, wasn’t investing enough. At that time, Instagram had grown to 1 billion users, about 40 percent of Facebook’s size, yet the photo-sharing app had only 1,000 employees compared to 35,000 employees at Facebook, he said.

“We were by far the fastest growing team. We produced the most revenue and relative to what we should have been at the time, I felt like we should have been much larger,” said Mr. Systrom, who is expected to testify for six hours.

Mr. Systrom said he found the decisions baffling. When asked by an F.T.C. lawyer why Mr. Zuckerberg might have decided to give Instagram fewer resources, Mr. Systrom said it was a consistent pattern during his tenure at Meta.

“Mark was not investing in Instagram because he believed we were a threat to their growth,” he said, referring to Mr. Zuckerberg’s prioritization of Facebook.