Not all converters are equal, but such conversion tools are not perfect. Since a PDF is essentially a picture (depending on the source document and how it was turned into a PDF in the first place), the conversion to Word usually involves converting recognizable text to regular text and anything else to a picture or part of one. For instance, bordered tables are typically converted to a combination of the text and a picture which shows the borders. This picture is the size of the page and sits on the graphics layer of the Word document. Furthermore, when the text is converted, you usually end up with extensive formatting: each line is turned into its own paragraph (sometimes text boxes are created), and line-spacing is often made into a precise number of points, rather than a blanket "single space" or similar formatting. Hence, the file size can grow very considerably.

You may find it easier, in this case, to copy the text several paragraphs at a time and use Paste Special to bring them into Word as "Unformatted text." If you can copy the pictures separately and bring them over as well, that would be the way to go.