When you create a Jira issue that contains header markup (e.g. "h3."), it looks completely different when viewed full page vs the two panel split view (as seen when browsing the backlog in the Scrum board).

H2s, for example, are large when viewed in the side details view (but lacks top margin) but the same H2 (on the same ticket) looks MUCH smaller when viewing the same issue full page.