Too many pictures on my phone

If you have "iCloud Photo Library" enabled in Settings > Photo & Camera, all photos will sync from iCloud to your iPhone.

See this page in the iPhone User Guide: http://help.apple.com/iphone/10/#/iph961b96c4d

By default, iCloud Photo Library will automatically enable "Optimize Storage" as well, meaning, that the iPhone will only download small, optimized versions for browsing. It will just look like all photos are stored on your iPhone, but actually the iPhone will download them from iCloud for browsing or editing. There will always just be as many photos on the iPhone as space permits. You will not get much storage back, if you disable iCloud Photo Library.

But do not delete any photos from your iPhone that you want to keep your computer or other devices. Deleting any photo from a device syncing with iCloud Photo Library will delete it from iCloud and your other synced devices as well.

If you prefer to disable iCloud Photo Library, just turn off iCloud Photo Library in Settings > iCloud > Photos, or Settings > Photos & Camera, see: Get help with your iCloud Photo Library - Apple Support

You can turn off iCloud Photo Library on a single device when you follow these steps:

• On your iPhone, iPad, or iPod touch, go to Settings > iCloud > Photos and turn off iCloud Photo Library.
• On your Mac, go to System Preferences > iCloud > Options next to Photos, then deselect iCloud Photo Library.
• On your Apple TV (4th generation), go to Settings > Accounts > iCloud and under Photos press Select to turn off iCloud Photo Library.

If you have Optimize Storage turned on, you might not have all of your original photos and videos on your device. To download the original photos and videos on your iOS device, go to Settings > iCloud > Photos and select Download and Keep Originals. On your Mac, open Photos, choose Photos > Preferences, then select Download Originals to this Mac. Then you can turn iCloud Photo Library off. You can also select the photos and videos that you want to download from iCloud.com.

Do not disable "Optimize Storage" if you do not have much free storage on your iPhone. Only after you disabled iCloud Photo Library on the iPhone delete the photos from your iPhone that you do not want on the device.

If it's a storage concern, you might want to consider uploading them to iCloud Photos or Google Photos. If you have previously synced pictures to iCloud Photos, deleting one on your iPhone will delete them from your other devices as well. Google Photos is great because it offers free unlimited storage. Dropbox works, too.

I'd think twice about giving my photos to Google. Their terms of use give them the right to mine your information and use with their partners, many of whom are advertising services. It's spelled out in Privacy Policy and discussed in speifics in this article: Uh oh. What did I just give Google?

I've heard, but not found confirmation, that they've used photos from Google Photos clients in some of their endeavors.

Additionally you might want to reconsider using Google services or Chrome as your browser after reading these:

Google Chrome Listening In To Your Room Shows The Importance Of Privacy Defense In Depth.

Unseen Burdens in Chrome That Can Lead a Mac to Lag

How to Manage the Secret Software That Google Chrome Installs on Your Mac\

Google Chrome's Sham 'Do Not Track' Feature

Maybe it is time you actually read that Google Drive Terms of Service ...

Google is known to "phone home" with users data as was found out with this new Google Photos service: Google Photos may be free — but there's still a cost.

Just some food for thought.

That's the reason you have all of your Mac's photos on your iPhone. The iCloud Photo Library will sync libraries between devices that are signed in to the same iCloud ID.

I don't use the iCPL so am not sure what will happen if you disable it on the iPhone. That's what you want to do if you don't want your other photos on the iPhone but I'm not sure how to do it. I'll see if I can get one of the contributors who are much more familiar with the iCPL to drop by and help.

Do you have the iCloud Photo Library enabled in the Settings ➙ Photos ➙ iCloud preference pane on your iPhone?