The 'data revolution' marks a time of growing interest and investment in data - big, small, or otherwise. Critical attention to data is also proliferating, exposing the diverse ways that data produces inequality of opportunity and harm in society. This paper draws the nascent field of critical data studies into conversation with emerging narratives in data-for-development (D4D) to advance the conceptualization of data inequalities, explaining how they both align with and diverge from core tropes of digital inequalities research - and why this matters for development. The paper examines the causes, consequences, and potential solutions to three 'data divides' - access to data, representation of the world as data, and control over data flows - through examples of digital identity systems and national data infrastructures, user-generated data, and personal behavioral data produced through corporate platforms. This understanding provides a basis for future research, practice, and policymaking on data-related (in)equalities in development contexts and beyond.