So yes, the planet can theoretically sustain billions more people but only if you look at planetary free square footage per capita, but at the cost of either them living in absolute poverty, or insane environmental damage and climate change which will also create poverty from the issues climate change will cause to agriculture and food prices, so we're back to millions living or dying in poverty.

Canada and Australia are huge countries but the livable parts are quite small. You can't have infinite growth on a planet of finite resources.

Lithuania isn't overpopulated. Japan sort-of is, most inhabitable territory is already built over. Egypt is definitely overpopulated, with 100 million people packed into inhabitable territory the size of Florida. Etc. etc.

Edit: to the downvoters, can you provide counterarguments? I would say that a good measure of overpopulation is that the country in question cannot possibly feed all of its population even if it tried very hard, and is inevitably dependent on imports.

Is it possible to feed more people? Yes, clearly, but mostly with highly processed junk food.

At the current population it would simply not be possible to feed everyone with a healthy diet. From my point of view, we've been past overpopulation for decades.