Help: Size of file was increased after upload versus to Document library

Hi all,

I have below trouble & looking forward to find out the solution.

[Pre-condition]
+ I created SPO Classic Team site
+ I want to upload my folder versus to Document library
+ In my PC, my folder structure like as the sample below

Parent Folder

|___Child Folder 01

|
|___file11.pdf - about 9.5 MB
|___file12.pdf - about 10 MB
|___file13.pdf - about 11 MB

|___Child Folder 02

|
|___file21.pdf - about 4 MB
|___file22.pdf - about 5 MB
|___file23.pdf - about 20 MB

[Actually]

+ After upload was done, I got the below result in SPO document library.
The file size of Child Folder 01 folder is increased, while Child Folder 02 is unchanged.

Parent Folder
|___Child Folder 01

|
|___file11.pdf - about 20 MB
|___file12.pdf - about 20 MB
|___file13.pdf - about 20 MB

|___Child Folder 02

|
|___file21.pdf - about 4 MB
|___file22.pdf - about 5 MB
|___file23.pdf - about 20 MB

[Question]
What is the cause of this problem? is there any way to fix it?

Thank you & looking forward to your support.

Nguyen Duc Hoa