Answer
They are smaller and have less mass.
Work Step by Step
With the same energy, a $\beta$ particle moves much faster than an $\alpha$ particle, and it is also smaller. That gives it more penetrating power.
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.