Today’s Question: It constantly baffles me how different the sizes can be between Lightroom Classic and, once you export something, on the hard drive. I recently had a photo in Lightroom Classic that showed as a big 44MB but once I exported it full size it was about 12MB as shown in the Mac Finder. It had been cropped a lot, if that makes a different. Please help me understand the difference and how to deal with it.
Tim’s Quick Answer: Especially if the original image in Lightroom Classic is a raw capture, if you export to a different file format the file size is very likely to differ significantly. Depending on the file type selected, the file can be considerably larger or considerably smaller. Cropping will also affect the file size, since it impacts the number of pixels included in the exported copy.
More Detail: Generally speaking, a raw capture will have a file size that is approximately equal in megabytes to the number of megapixels for the image sensor. There are some factors that can impact this relationship such as compression of the raw file, but in general the file size will be relatively close in this way.
When you export an image from Lightroom Classic, you’ll generally be exporting to a file format other than the original, unless you’re simply exporting a copy of the original raw capture without adjustments applied to it. The file format and settings can have a significant impact on the file size.
If you export as a JPEG for sharing online, for example, the file size will be considerably smaller than the original because of the use of lossy compression with JPEG images. This would be compounded by cropping the image to reduce the total number of pixels. If you were to resize the image to smaller pixel dimensions the file size would be smaller still.
If you export in a format such as TIFF, the file size will generally be significantly larger. For example, all other things being equal, a TIFF file without layers or compression will be about three times larger than the original raw capture file. This is because most raw captures only contain one color value for each pixel, while a TIFF image will have full RGB color values for all pixels.
Because of all the variables involved, I don’t consider file size to be a good placeholder for image resolution or pixel dimensions. Furthermore, these factors make comparing file sizes for different file formats to be problematic. Instead, I recommend focusing on the purpose of the file format. If you’ll be printing an image, for example, the TIFF format is a good choice for maintaining image quality, at the cost of a larger file size. For online sharing a JPEG image is a good choice because it provides a much smaller file size though with some degree of lost image quality.
