There is no way to gess here what is that you have and what is the configuration or usage you need.
I don't think "convert" is a proper word for generating a pdf. Here is why.
PDF is not a "working" file, it is an output file. And there is not one "flavor" of pdf but can be customized according to your target needs.
Among the things you can configure your file that affects the file size are:
1) Color mode for images. If you used RGB files and you need a press ready file will be converted to CMYK. This files are bigger.
2) Resampling of the images. If the images used are bigger than ppi you need. This actualy can reduce a little the output file.
3) Compression. Indesign file size does not take into acount the external images. And thoose images can be a verey compressed jpg photos for example. On the output pdf they are inside and re-compressed as needed. But you should NOT use jpg compression on the press quality. The recomended compression is none or ZIP, which compresses less than the original jpg.
4) Converting text to curves. Instead of having text, you have thousands of curves which ocupies more space.
I'm adding another thing here.
If you are preparing a file for print file size does not matter.
If you need to send the file for email, play with thoose options.