The font size of .form-control in .input-group-sm is different from a single .form-control-sm when customize $input-font-size-sm or $input-btn-font-size-sm. The same is true for lg and .btn.
$input-font-size-sm: .5rem;
$input-font-size-lg: 2rem;

Demo: https://codepen.io/anon/pen/PyMyKo
One cause is related to the following lines:
|
font-size: $input-font-size-sm; |
|
font-size: $font-size-sm; |
The font size of
.form-controlin.input-group-smis different from a single.form-control-smwhen customize$input-font-size-smor$input-btn-font-size-sm. The same is true forlgand.btn.Demo: https://codepen.io/anon/pen/PyMyKo
One cause is related to the following lines:
bootstrap/scss/_forms.scss
Line 145 in 4b15ec9
bootstrap/scss/_input-group.scss
Line 149 in 4b15ec9