There is no scale slider or entry where you can manipulate image or object size by a scaling factor (e.g. 1.25). Though no equivalent exists in Preview.app, it will allow you to manipulate the dimensions and dpi, while displaying the scaling percentage and storage size.
In Pages, single-click the object. Edit -> Copy. Now launch Preview, and File > New from Clipboard.
Menu > Tools > Adjust Size ...
Export as new image object, and replace back in Pages.
Same as above, except you can't use the Adjust Size until it the Shape is exported as an image object, then reopened in Preview.
Another option, outside of Preview, is a simple Automator application. Drag and drop (or interactively) choose a file for scaling, open the file, make a copy of it in a resized folder, and then be prompted for the percentage of scaling you wish to apply. Then put the finished image back in Pages.
The following article provides the steps to create an Automator application that does the above on single or multiple files. On the next to last bullet, where you deal with the scale image action, the selection offers pixels or percentage. You also want to select Options at the bottom of the Scale Image action, and enable show this action when the workflow runs. This will make this dialog pop-up so you can modify the scaling factor.
I named my Automator application Resizer and placed it on the desktop. I created a folder called Resized, and also placed it on the desktop, for convenience. I did not use the optional steps offered in the article.