You will get a much more direct answer if you can upload your model here. If the model file is smaller than about 15MB, drag and drop the file into your next post from Windows Explorer (or Finder on Mac).
If it is bigger, share it via Dropbox, OneDrive or WeTransfer and share a link to the file here.
Two likely causes of the large bounding boxes have already been mentioned:
1. Axes outside the geometry of a group or component, or not aligned with the object’s geometry.
It’s easy to change a component’s axes by selecting the component then using R-click/Change Axes.
It’s possible but not so simple to change a group’s axes, so I’d recommend making each group into a component first, then change the component axes as above. Give it a meaningful name when you create it.
2. Stray geometry within the component (or group) some distance from the object’s geometry.
Open the object for editing (double click on it, or R-click/Edit (component or group). Select using a left-to-right window all of the object you want to keep. Cut the selection to the clipboard. Select all of what’s left, using the keyboard short Crtl+A (Windows) or Cmd+A (Mac) then delete it using the backspace or delete key, or R-click/Erase.
Then Edit/Paste in place the object contents back into the component or group.
Close the object, check and if necessary change its axes.
Both of these have already been described but I thought it might be helpful to summarise.