Short-form content on social media has transformed user engagement, but its cognitive and informational implications remain underexplored. Drawing on Cognitive Load Theory and dual-process models, this study examines how content length influences cognitive processing, engagement patterns, and information quality. Analyzing YouTube comments with the AIMQ framework, readability scores, and sentiment analysis, we find that shorts spark briefer, more emotional and fragmented exchanges, whereas long-form videos elicit deeper, more structured discourse. T-tests and ANOVAs confirm significant gaps in cognitive depth and information quality between the two formats. These findings highlight the trade-offs between engagement and cognitive processing in digital media, offering insights for content creators, marketers, and platform designers.