Online donation platforms, such as DonorsChoose, GlobalGiving, or CrowdFunder, enable donors to financially support entities in need. In a typical scenario, after a fundraiser submits a request specifying her need, donors contribute financially to help raise the target amount within a pre-specified timeframe. While the goal of such platforms is to counterbalance societal inequalities, biased donation trends might exacerbate the unfair distribution of resources to those in need. Prior research has looked at the impact of biased data, models, or human behavior on inequality in different socio-technical systems, while largely ignoring the choice architecture, in which the funding decisions are made. In this paper, we focus on (i) quantifying inequality in the project funding in online donation platforms, and (ii) understanding the impact of platform design on donors' behavior in magnifying those inequalities. To this end, we borrow decomposable measures of income inequality from economics, and apply it to identify candidate factors contributing to inequality on the DonorsChoose website. Analyzing longitudinal changes in the website design, we show how the platform design impacts the relative contribution of the different factors. Our work motivates the need for a careful investigation of the impact of choice architectures on user decisions, in donation platforms in particular, and in online platforms more generally.