PayPal Holdings Inc. beat estimates with its quarterly results Monday, but its shares were headed lower in after-hours action as Wall Street unpacked how much of PayPal’s growth is coming from a lower-margin part of the business.

While PayPal PYPL executives boosted their profit forecast for the full year and spoke of more upbeat revenue expectations, they also now project at least 100 basis points of operating-margin expansion this year. The company’s prior outlook was for about 125 basis points.

PayPal...