Issue values dictionary

[claire morandin]

 I have two text file with a bunch of transcript name and their corresponding length, it looks like this:
ERCC.txt
ERCC-00002	1061
ERCC-00003	1023
ERCC-00004	523
ERCC-00009	984
ERCC-00012	994
ERCC-00013	808
ERCC-00014	1957
ERCC-00016	844
ERCC-00017	1136
ERCC-00019	644
blast.tx
ERCC-00002	1058
ERCC-00003	1017
ERCC-00004	519
ERCC-00009	977
ERCC-00019	638
ERCC-00022	746
ERCC-00024	134
ERCC-00024	126
ERCC-00024	98
ERCC-00025	445

I want to compare the length of the transcript and see if the length in blast.txt is at least 90% of the length in ERCC.txt for the corresponding transcript name ( I hope I am clear!) 
So I wrote the following script:
ercctranscript_size = {}
for line in open('ERCC.txt'):
    columns = line.strip().split()
    transcript = columns[0]
    size = columns[1]
    ercctranscript_size[transcript] = int(size)

unknown_transcript = open('Not_sequenced_ERCC_transcript.txt', 'w')
blast_file = open('blast.txt')
out_file = open ('out.txt', 'w')

blast_transcript = {}
blast_file.readline()
for line in blast_file:
    blasttranscript = columns[0].strip()
    blastsize = columns[1].strip()
    blast_transcript[blasttranscript] = int(blastsize)
    
blastsize = blast_transcript[blasttranscript]    
size = ercctranscript_size[transcript]
print size 
if transcript not in blast_transcript:
    unknown_transcript.write('{0}\n'.format(transcript))
else:
    size = ercctranscript_size[transcript]
    if blastsize >= 0.9*size:
        print >> out_file, transcript, True
    else:
        print >> out_file, transcript, False

But I have a problem storing all size length to the value size as it is always comes back with the last entry. 
Could anyone explain to me what I am doing wrong and how I should set the values for each dictionary? I am really new to python and this is my first script 

Thanks for your help everybody!