PNG is a familiar lossless format that works well for graphics and transparency. WebP is more flexible for modern web delivery and can offer useful compression.
What should you consider?
Test the actual image and destination rather than assuming one format always produces the smallest file.
Practical workflow
Start with the destination requirement, make a copy rather than overwriting an important original, and compare the output at the size where it will actually be used. If the file is still too large, consider whether resizing, compression or a different format solves the real problem.
Related tool
Tip: The smallest file is not automatically the best file. Compatibility and visible quality matter too.