File size is not memory use
A DOCX is compressed. Opening its ZIP structure, inflating media and decoding one image into pixels can use much more memory than the file size on disk. A 24-megapixel photo alone needs roughly width × height × four bytes for an RGBA canvas, before encoded input and output buffers are counted.
There is no reliable multiplier for every document because image dimensions, compression ratios, browser implementation and output settings differ.
Reduce peak pressure
Use a 64-bit desktop browser, close other tabs, connect power and keep free system memory available. Sequential processing lowers the number of decoded images held at once. Avoid converting every image to a Data URL because Base64 adds overhead and creates large strings.
Recognize failure modes
A tab disappearing, a browser out-of-memory message or a long stall may indicate memory pressure. Retry with a smaller document, split the Word file, reduce other workloads or use desktop software. Never delete the original until the generated DOCX opens and key pages are checked.