I'd say that 5Mb is large for a SKP, 25Mb is enormous.
An OBJ file is simply a special kind of text file describing all of the faces and their materials.
Its size comes from its source's complexity.
What do you think is making your SKP so large?
Are you using Model Info > Statistics > Purge Unused... and Fix... at least that will optimize things...
If you have lots for facets or large image files that too will bump up the size.
So for example don't use 3d trees with every leaf modeled, unless you want an overhanging branch next to the camera - but even then an image 'cutout' as scenery will be much smaller and efficient, and possibly give better results.
The exporter and the renderer will need to process every leaf and twig even if they are never going to be even a pixel in the final image!
The only way to reduce the OBJ file size is to export less 'geometry'.
The Exporter only exports what is 'visible' - that is it ignores any geoemtry/groups/instances that are hidden OR more usually on layers that are switched 'off'.
So to reduce complexity your model should be layered appropriately [all 'raw' geometry should be made/put on Layer0, and then that geometry made into groups/components, which then have their own layers - remember SKP layers only control visibility so never layer 'raw' geometry that connected as even when some geometry is unseen it can be affected by changes to visible connected geometry with unexpected consequences].
Then you can switch off unwanted objects [perhaps with some purpose-made scene-tabs changing layers but not changing other settings like camera] and then export what is needed for your render... For example if you are rendering the outside of your house why do you need to export the interior fittings in the bathroom or the car inside the locked garage? Perhaps you'd see a sofa through the window but not the WC or the car! Conversely if you are rendering an interior view of the kitchen why do you need to export all of the external objects - perhaps you see part of the site out of the rear window but not the front street with the parked cars and avenue of trees... If you are rendering a car in the middle distance why include the gages on the dash or spare-tire in the trunk if you'll never see them in your render.
Every piece of unneeded geometry you have visible in the SKP gets translated into an entry in the OBJ and thence into the OCS, even if the OBJ is a manageable size the OCS will take ages rendering as it decides which bits to 'ignore'...
You can make different OBJ files for different aspects of your SKP 'visible' and then use them all [linked if desired] within the same OCS file...