col-sm in Bootstrap 4 allows it to automatically consume the space that remains. My guess is you've a CSS conflict coming from somewhere that's overriding Bootstrap 4 CSS. I don't see how this could be specific to CB Activity though as it's not using a grid for its activity display and is most likely the canvas layout styling around it. You can apply any CSS fixes you need using CBs template CSS overrides shown below.
Kyle (Krileon) Community Builder Team Member Before posting on forums:
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krileon
