It's just a default limit. The limit originally comes from the Facebook API, where it is 100. JIG also adds a limit of 100 to Flickr. Recent posts source has a limit of 50. The reason behind the limit is to not overwhelm the site with an unlimited number of thumbnails, on the first try. Some accounts at the external sources have a lot of content.
To show everything you need, increase the limit by adjusting the Shortcode Editor > General tab > Thumbnail count and dimensions section > Limit image count setting to a high enough number. Entering 0 means maximum amount which is 500 for external sources.
If you don't use presets in your shortcodes then you can also choose a global limit when setting the limit on the JIG settings screen.
Tip: You can also change the preset authority to exclude the Limit image count setting from presets.
The RSS feeds content amount is not limited by JIG, rather by the feed itself. JIG looks at the feed as an XML document and loads the content that is present in it at any given time. Unlike traditional RSS readers it does not accumulate entries over time.
Previously the default limit was 25 across most sources.