The theme is most likely altering the main query via the
pre_get_posts hook, chaning the
posts_per_page argument to -1 for all posts, or another higher number.
You can search your theme folder for
pre_get_posts to see all the times this hook is used.
A better solution would be to install the Query Monitor. With this plugin installed and active, you get a wealth of information about each page request.
To use it, login as admin and visit your homepage once the plugin is active. The admin bar will have a new options. Click the new option (looks like a bunch of numbers) and choose the "Hooks & Actions" section. From there, select "Pre" from the "Hook" dropdown and it will list all where the
pre_get_posts hooks is being called.