Why does using merge function in two different dataframes results me more rows?

I have two dataframes of the shape: (4000,3) (2000,3) , with the bellow info and cols:

df1:

imo	speed	length
1	1	4
1	2	4
2	10	10
2	12	10

df2:

imo	dwt	name
1	52	test1
2	62	test2
3	785	test3
4	353	test4

i would like to add column dwt of df2 to df1 based on the same imo.

imo	speed	length	dwt
1	1	4	52
1	2	4	52
2	10	10	62
2	12	10	62

but when i am trying to do pd.merge(df1,df2, on = 'imo', how = 'inner') , the result is much more rows than the original shape of df1 how is that possible?

from Why does using merge function in two different dataframes results me more rows?