Civic technologies have the potential to support participation and influence decision-making in governmental processes. Digital participatory budgeting platforms are examples of civic technologies designed to support citizens in making proposals and allocating budgets. Investigating the empirical case of urban biking activists in Madrid, we explore how the design of the digital platform Decide Madrid impacted the collaborative practices involved in digital participatory budgeting. We found that the design of the platform made the interaction competitive, where individuals sought to gain votes for their single proposals, rather than consider the relations across proposals and the larger context of the city decisions, even if the institutional process rewarded collective support. In this way, the platforms' design led to forms of individualistic, competitive, and static participation, therefore limiting the possibilities for empowering citizens in scoping and self-regulating participatory budgeting collaboratively. We argue that for digital participatory budgeting platforms to support cooperative engagements they must be revisable and reviewable while supporting accountability among participants and visibility of proposals and activities.