According to this guide the big or small dots just identify different types of fish. Note: the top comment in the thread has a formatted table for easy reference.
Eg. "Allural Sea Bass" will always be a Large Yellow dot. In general, fish indicated by the large icon (dot with a circle around it) are more challenging than the other fish. Fish indicated by just a dot are typically easier to catch.
The linked guide gives specific lures to use on each fish, but some fish will be attracted by multiple types or simply by any lure. It's a good guide and I've used it quite a bit, because the official guide is severely lacking in fishing details.
Since you mentioned you're just trying to get to level 10 fishing, it would be advantageous to target the large dots - both blue and yellow. They offer more SP (Skill experience points) than the smaller fish, and you'll likely catch a few small fish before the big one takes the bait anyway.