Does my proposal have higher chances of being approved if I propose a short programme?

No, the only key criterion for approval is the excellence and the feasibility of your idea. The ESO panel members are instructed to judge only the merit of your scientific idea and if your scientific justification is convincing. Thus, when writing your proposal, you can try to identify the best strategy to optimize the use of the telescope time, but you should not try to reduce the observing time if this means de-scoping your proposal.  Ask what you need for achieving your scientific goals: if you need 3 hours, ask 3 hours, if you need 70, ask 70.