What feature makes Mars's past habitability inherently more vulnerable to collapse than ancient Venus, based on atmospheric mass?

Its thin atmosphere, making it susceptible to changes in solar output or orbital dynamics

A crucial difference affecting long-term stability is the mass of the respective atmospheres. Venus possesses a massive atmosphere, which provides significant thermal inertia, meaning once a climate is established, it is slower to change catastrophically. Mars, conversely, has a very thin atmosphere. This low atmospheric mass makes Mars highly susceptible to external variables, such as fluctuations in solar output or minor changes in orbital dynamics, which could easily push its climate past tipping points, causing past water bodies to freeze or sublimate quickly. While Venus's breakdown was catastrophic, the scale of its potential past oceans was likely much grander due to its massive atmospheric reservoir providing more substantial initial support.