Backstage tab icons ignore BarImageSize and show always small size

I create RibbonBackstageViewModel that contains couple of pages with appropriate icons:

    private RibbonBackstageViewModel CreateBackstage(BackstageCommands commands) => new()
    {
        Items =
        {
            new BackstageHomeTabViewModel()
            {
                SmallIcon = _imageProvider.GetImage(BackstageControlKeys.BackstageHome, BarImageSize.Small)
            },
            new BackstageNewTabViewModel()
            {
                SmallIcon = _imageProvider.GetImage(BackstageControlKeys.BackstageNew, BarImageSize.Small)
            },
            new RibbonBackstageHeaderButtonViewModel(BackstageControlKeys.BackstageOpen, "Open", commands.OpenCommand)
            {
                SmallIcon = _imageProvider.GetImage(BackstageControlKeys.BackstageOpen, BarImageSize.Small)
            },
    }

No matter which icon size I request: BarImageSize.Small, BarImageSize.Medium or BarImageSize.Large, the backstage shows always small icon.
I checked that _imageProvider delivers bitamps of appropriate size.

I tried as well to initialize RibbonBackstageTabViewModel.LargeIcon and RibbonBackstageHeaderButtonViewModel.LargeIcon rather than SmallIcon but it does not work either.