A quality setting is not a file-size promise
A photograph with texture, noise, or small details can need more bytes than a flat graphic at the same quality setting. Use a specific KB target when a form has a ceiling, and compare the actual result instead of treating the slider as a percentage of file-size reduction.
Choose the family that fits your source
JPEG presets accept JPG sources and produce JPEGs. Mixed-format presets accept JPEG, PNG, and WebP, with a choice of output. If the image has transparent areas, select an output that can keep them. JPEG fills those areas white, which may matter more than a small difference in bytes.
When resizing is the better first step
If a large camera photo is being prepared for a small display box, many source pixels may be unnecessary. Use dimension-first reduction to limit that resolution before compression. If exact dimensions are mandatory, resize to those dimensions and then recheck them after any target-size compression.