The scales are completely different. You do not use arts as you do abilities. That the numbers are larger for arts do not mean they are easier. All it does is give a finer gradation for the difficulty of spells.
If you want to compare skills and arts, compare the amount of experience:
15 XP, Skill 2, Art 5: the minimum for training someone (for both arts and skills)
30 XP, Skill 3, Art 7: Basic skill, you can cast 3rd magnitude spells
105 XP, Skill 6, Art 14: Skilled, you can cast 6th magnitude spells
225 XP, Skill 9, Art 20: Master, 8th magnitude spells - becoming an archmagus involves creating a 7th magnitude spell, IIRC.
...
855 XP, Skill 18, Art 40: Theoretical maximum
The main difference is that magi tend of course to have good books and much, much longer lives than mundanes.
You also get a lot more experience than in 4th edition, whether you are a magus or not, and virtues to get even more are easily available. Not to mention that flaws are nicer overall than they were in previous editions.
Did you check the conversion guidelines at the end of the book? They might help clarify things a bit.
Skills in ArM5 are broader. For combat, the rationale is that warriors didn't "just" learn to fight with a sword, but learned the usage of shields and various other weapons. Honestly, there is not much lost in not having a separate skill for individual weapons. There are enough sinks for experience as it is.
If you fight with swords, pick a 'sword' specialization.
Have you checked the design notes posted on the ArM5 page, in the column at the right?
Welcome aboard, by the way.