You have a few issues that the planet and its residents need to confront
Size of ant: average 10mm
Size of human: average 1650mm
For your giant to be same ratio from a human, your giant's size: 272.5m high
Due to the Square Cube Law (volume / weight and surface area increase at different rates as size enlarges) you run into issues when you have giants. This is why elephant look so ponderous, heavy and flabby. They aren't simply 'scaled up' versions of smaller creatures.
The reason is for elephants as they grow larger, their weight increases at a much greater ratio. More weight = more blood flow pressure required to service it, more food, more stretchy skin, more force required to move it. Hence slow and heavy.
It also affects our skeletons. Bones would break under such force as your giant would endure.
So we need to eliminate the effect of weight - let's just say we need then a much lower gravity planet, weak enough such that weight is not so much of an issue.
The other factors are now more physiological. Blue whales are 30m in length, the largest currently on the planet: why aren't they larger? The answer: energy. Mammals use a lot of energy to maintain various systems in our bodies, 10 times more than a reptile. Your giant, if like us, would have massive metabolic issues to maintain the services required to keep body temperature, heart rate and flow, muscle tone and oxygenation, constant. So I would imagine you would need a high level of oxygen in the air, perhaps a higher air pressure, a fairly constant warm temperature planet to reduce warmth required for our giant, and not much in the way of mountains for the giant to climb.
You need heaps of it, and this is a major determinant of size. This planet needs to be completely filled with low energy consuming food. Blue whales eat krill, basically they don't need to chew, they just swallow and the krill turn into easily absorbable food for their bodies, your giants would likely be shovelling food down constantly and the food needs to not require to many resources to convert into essential fats, vitamins, proteins and other essentials.
And gestation. To get to maturity, your giant needs to grow from a single cell, to a full size adult. This severely limits the size of animals on our planet - your giant would not just take decades to get to maturity, but likely centuries. The planet would need to have been there for a very long time to enable a lifecycle where this size and length of lifespan is beneficial for the giants species.