The real problem is saying "yes we can do it all in that timeline" instead of saying "That's fine, but we won't get to everything. We will work with you to prioritise the most important things to get done in that time."

I know it's painful, but just hold that line. It's not lying or negotiating or padding; in fact it may be the only true statement you can say.

If a civil engineer were building a bridge, and couldn't do it safely in the budget and timeline, they'd say so, and the project manager would nod and diligently write that down. We have a more subservient relationship with project managers in software because we choose to.