1600 × 900
The raster grid: how many pixels wide and high the image is.
DIMENSIONS ≠ FILE WEIGHT
A smaller image can still be the larger file.
Width and height tell you how many pixels the image contains. KB and MB tell you how much encoded data the file occupies. Those measurements are related, but they are not interchangeable.
420 KB
The amount of encoded storage or transfer data the file uses.
800 CSS px
How large the browser or application actually renders it on screen.
WHY THEY DIVERGE
Four things can outweigh the raw dimensions.
PNG and JPEG store the same scene differently.
A photographic PNG can be much heavier than a JPEG or lossy WebP of the same dimensions because PNG is lossless while photographic lossy formats discard information to save space.
Two 1200 × 800 files can have very different weights.
One export may use aggressive compression and another may prioritize quality. Same pixel grid, different encoded file.
Busy detail is harder to compress.
Grass, hair, confetti and fine texture tend to require more data than a flat wall, soft gradient or simple graphic at identical dimensions.
Transparency and encoding choices matter.
Some formats store alpha transparency and different color information. Those capabilities can change the file weight independently of width and height.
SAME DIMENSIONS, DIFFERENT COMPLEXITY
Pixels that are difficult to predict need more data.
A simple image with large uniform areas can compress efficiently. A highly detailed scene changes from pixel to pixel and is harder to represent compactly. That is why file size cannot be predicted accurately from dimensions alone.
WHAT TO CHANGE
Use the control that matches the problem.
ProblemChange dimensions?Change compression/format?
Image is physically too wide for upload requirementsYesMaybe
Dimensions are correct but file exceeds MB limitNot necessarilyUsually
Website displays a huge source in a small cardYesAlso useful
PNG photo is unexpectedly heavyMaybeStrong candidate
TWO DIFFERENT OPERATIONS
Resize changes the grid. Compression changes how that grid is stored.
Reducing dimensions throws away pixels because the output contains a smaller raster grid. Compression works on the encoded representation. Lossy compression may also discard image information, while lossless compression preserves it.
MDN distinguishes lossy formats such as JPEG from lossless PNG and notes that WebP can operate in either mode. That is why “make the dimensions smaller” and “make the file lighter” are related but separate tasks.
