Two Separate Levers: Dimensions and Quality
Reducing an image's file size can come from two genuinely independent adjustments, and conflating them leads to worse results than necessary. Resizing reduces the actual pixel dimensions (width x height) — fewer total pixels means less data to store, full stop, regardless of compression settings. Compression quality (for lossy formats like JPEG or WebP) keeps the same pixel dimensions but discards more visual detail per pixel through more aggressive quantization — a separate axis entirely from dimensions.
The most effective size reduction usually combines both, in the right order: resize first to the dimensions actually needed for display (a full-resolution photo displayed at 400px wide is carrying pixels nobody will ever see), then apply compression quality adjustment to the already-smaller image — compressing first and resizing after wastes the compression work on pixels about to be discarded anyway.
Locking aspect ratio while resizing avoids an easy, common mistake: an unlocked resize that doesn't preserve the original width-to-height proportion visibly stretches or squashes the image, a distortion that's obvious and unflattering on virtually any photo.