Background: Obesity and insufficient physical activity are major global health risks, but their population-level associations with life expectancy may overlap because both evolve with economic and epidemiological transition. Objective: To compare the strength and robustness of the associations of adult obesity prevalence and insufficient physical activity with life expectancy at birth in a longitudinal ecological panel of countries. Material and Methods: Annual country-level data for 2000-2020 were analysed. The full source panel contained 4,095 country-year observations from 195 countries, and the primary complete-case sample included 3,365 observations from 172 countries. Life expectancy at birth was the outcome. Adult obesity prevalence and insufficient physical activity were the main exposures. Models included two-way fixed-effects specifications with country-clustered standard errors, oneyear lagged exposures, a joint lagged specification, country-specific time trends, a non-linear obesity model, a smoking sensitivity analysis, and three-year and five-year lag sensitivity models. Results: In standalone lagged models, obesity (β = -0.3017, p < 0.001) and insufficient physical activity (β = -0.0716, p = 0.020) were inversely associated with life expectancy. In the joint lagged model, obesity remained significant (β = -0.2873, p < 0.001), whereas insufficient physical activity attenuated to non-significance (β = -0.0153, p = 0.643). In the country-specific time-trends model, obesity remained significant (β = -0.2496, p = 0.040), but insufficient physical activity did not (β = 0.1325, p = 0.457). Three-year and five-year lag models supported this pattern. Conclusions: In this country-level panel, adult obesity prevalence showed a more robust inverse association with life expectancy than insufficient physical activity. The findings should not be interpreted as evidence that obesity alone causes lower life expectancy. Rather, obesity may represent a cumulative marker of overlapping metabolic, social, environmental, and policy-related processes that shape population longevity. Supplementary information: The online version contains supplementary material available at 10.32394/rpzh/225361