how to merge two files while preserving the number of rows of one file in merged one?

Hello,

head(a)

           ID_1 pheno
1             0     B
2 fam1000_G1000     0
3 fam1001_G1001     0
4 fam1003_G1003     1
5 fam1005_G1005     0
6 fam1009_G1009     0

head(b)

           ID_1          ID_2 missing
1             0             0       0
2 fam1000_G1000 fam1000_G1000       0
3 fam1001_G1001 fam1001_G1001       0
4 fam1003_G1003 fam1003_G1003       0
5 fam1005_G1005 fam1005_G1005       0
6 fam1009_G1009 fam1009_G1009       0

dim(b)

[1] 1602    3

dim(a)

[1] 1652    2

m=merge(a,b,by="ID_1")
dim(m)

[1] 1499    4

head(m)

          ID_1 pheno         ID_2 missing
1            0     B            0       0
2 fam0110_G110     1 fam0110_G110       0
3 fam0117_G117     1 fam0117_G117       0
4 fam0124_G124  <NA> fam0124_G124       0

I would like my merged file (m) to have the same number of lines like
(b), that is 1602. Can you please let me know how would I do that?

Thanks
Ana