UIStackView does not respect size of programmatically added buttons

I have a UIStackView that includes either three or four custom UIButtons. All buttons are completely rounded, the left and right buttons are slightly bigger than the center one/two. Somehow, I can't get UIStackView to respect the bounds set by DeckPageButton.

Here's the function in my VC that renders the buttons:

    private func setupButtons() {
        guard let deck = self.deck else {
            log.error("Failed to load deck")
            return
        }
        
        let nFloat = CGFloat(deck.type.buttons.count)
        let rad = (deckButtonsStackView.bounds.size.width - (8.0 * (nFloat - 1.0))) / nFloat
        for type in deck.type.buttons {
            let button = DeckPageButton()
            button.setupView(type: type, size: CGSize(width: rad, height: rad))
            button.translatesAutoresizingMaskIntoConstraints = false
            deckButtonsStackView.addArrangedSubview(button)
        }
    }

And here's the setupView function:

func setupView(type: DeckPageButtonType, size: CGSize) {
        bounds.size = size.resized(multiplier: type.radiusCoefficient)
        layer.cornerRadius = size.height / 2
        layer.borderWidth = 5
        layer.borderColor = type.borderColor
    }

I tried different Alignment and Distribution options for the deckButtonsStackView and none of them respect the

1. aspect ratio of the Buttons - they are not square, hence also not circular
2. edge size - all buttons are the same size even though the inner ones set their boundsto a smaller scale

Here's the best outcome, which I've had with FillEqually (same code for three and four button options):

I've also tried setting the frame of the buttons instead of bounds and keeping everything else the same, this was the outcome:

Is UIStackView just a bad choice for my goal? If so, what would be a better approach?

You should use constraints to set the size of each button:

In setUpView, instead of bounds.size = ...

let size = size.resized(multiplier: type.radiusCoefficient)
NSLayoutConstraint.activate([
    widthAnchor.constraint(equalToConstant: size.width),
    heightAnchor.constraint(equalToConstant: size.height)
])

After that, setting the spacing to some reasonable value, alignment to "Center", and distribution to "Equal Centering" should give you a desired result.