Steps to reproduce

Build and run on MacOS desktop any CupertinoApp with a widget with stacked checkboxes, such as:

Column(children: [
  CupertinoCheckbox(value: true, onChanged: (){}),
  CupertinoCheckbox(value: true, onChanged: (){}),
  CupertinoCheckbox(value: true, onChanged: (){}),
])

Observe that the three stacked checkboxes have excessive vertical spacing, and no means of reducing it (no parameters dev can pass to constructor will reduce vertical spacing).

This is caused by:

https://github.com/flutter/flutter/tree/main/packages/flutter/lib/src/cupertino/checkbox.dart

Line 433: 
  size: const Size.square(kMinInteractiveDimensionCupertino),

Where constants.dart is:
  const double kMinInteractiveDimensionCupertino = 44.0;

That causes minimum size of buildToggleable() result to be the minimum tap-target size for iOS, which is the wrong standard to be applying if Platform.isMacOS.

Note that this default behavior of CupertinoCheckbox is not even internally consistent with behavior of CupertinoRadio where minimum is set as:

const Size _size = Size(18.0, 18.0);

Expected results

Minimum spacing for checkboxes set consistent with a desktop-OS platform with precision pointing device.

Actual results

Minimum spacing is set for "stab at it with a fat finger" iOS standards, and cannot be reduced.

Suggested Fix

I've forked the checkbox.dart file and find that using the CupertinoRadio's default value of 18 instead produces the best results, letting the developer optionally add additional padding around the Widget if needed:

const minCheckboxSquareSizeMacOS = 18.0;

Making the line in checkbox build():

  size: const Size.square( Platform.isMacOS ? minCheckboxSquareSizeMacOS : kMinInteractiveDimensionCupertino ),