To add to @connie1018's explanation:
It's not your SP decrease, it is your VP (Voting Power).
If you would like a more graphical illustration, steemworld.org/@honjun88 would be a neater choice.
Looking at your account, because your SP is so low, it yields no effect on your voting. You only contributed a "thumbs up", while you are not helping the reader neither yourself (curation) in the long run.
Therefore, Connie's summary on the need to Power up and her illustration is very important for newbies like you. :)
Thanks @littlenewthings for the link! That helps to keep it in check. Previously I was using another site. So if I've powered up in the sense where my voting power is of higher value, because my SP is high, does the SP reduce in value over time? Due to lack of activity, etc? Hence my voting power also will reduce over time?